diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 37ade01..0fffe20 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -88,6 +88,17 @@ jobs:
           python-version: "3.14"
       - run: python3 meta/scripts/check_go_work.py
 
+  adr-numbers-check:
+    name: ADR number uniqueness check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
+        with:
+          # renovate: datasource=python-version depName=python
+          python-version: "3.14"
+      - run: python3 meta/scripts/check_adr_numbers.py
+
   secrets-check:
     name: Secrets check
     runs-on: ubuntu-latest
diff --git a/.vscode/settings.json b/.vscode/settings.json
index 59b7c2a..ed22bb7 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -52,6 +52,9 @@
     "check: go work": [
       "**/go.mod",
       "go.work"
+    ],
+    "check: adr numbers": [
+      "**/docs/adr/*.md"
     ]
   },
   "triggerTaskOnSave.showStatusBarToggle": true,
diff --git a/.vscode/tasks.json b/.vscode/tasks.json
index 213e46f..4d2e486 100644
--- a/.vscode/tasks.json
+++ b/.vscode/tasks.json
@@ -62,6 +62,33 @@
         }
       }
     },
+    {
+      "label": "check: adr numbers",
+      "type": "shell",
+      "command": "python3",
+      "args": ["${workspaceFolder}/meta/scripts/check_adr_numbers.py"],
+      "presentation": {
+        "reveal": "silent",
+        "panel": "dedicated",
+        "showReuseMessage": false,
+        "clear": true,
+        "revealProblems": "onProblem"
+      },
+      "problemMatcher": {
+        "owner": "check-adr-numbers",
+        "source": "check_adr_numbers",
+        "fileLocation": ["relative", "${workspaceFolder}"],
+        "severity": "error",
+        "pattern": {
+          "regexp": "^(\\S+):(\\d+):(\\d+)-(\\d+):\\s+(.+)$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "endColumn": 4,
+          "message": 5
+        }
+      }
+    },
     {
       "label": "Bazel Build all",
       "detail": "bazel build //...",
diff --git a/CONTEXT-MAP.md b/CONTEXT-MAP.md
index c388579..ebaf12a 100644
--- a/CONTEXT-MAP.md
+++ b/CONTEXT-MAP.md
@@ -5,7 +5,8 @@ directories in the [README](README.md), not a directory under `src/`. See
 [`docs/agents/domain.md`](docs/agents/domain.md) for how the engineering skills consume this file.
 
 Each context owns its own `CONTEXT.md` (its glossary) and its own `docs/adr/` (decisions scoped to
-it). Repo-wide decisions live in `docs/adr/`.
+it). Repo-wide decisions live in `docs/adr/`. ADR *numbers* are unique across all of those
+directories rather than per directory — see [`docs/agents/domain.md`](docs/agents/domain.md).
 
 ## Contexts
 
@@ -27,8 +28,7 @@ it). Repo-wide decisions live in `docs/adr/`.
   Treat a request for anything under "planned" as unimplemented: a redundancy check that matched a
   request against this domain language would otherwise close a real one as already-done.
 
-- **Repo meta** — `meta/`
-  _(`CONTEXT.md` not yet written)_
+- **Repo meta** — `meta/` ([`CONTEXT.md`](meta/CONTEXT.md))
 
   The monorepo's own automation: `meta/scripts/` (pre-commit checks, changed-path classification,
   Renovate proposal ratification and manual-job triggering, base-image pin sync) and
diff --git a/docs/agents/domain.md b/docs/agents/domain.md
index 7c5c608..9a8548e 100644
--- a/docs/agents/domain.md
+++ b/docs/agents/domain.md
@@ -10,6 +10,28 @@ codebase.
 - **`docs/adr/`**: system-wide architectural decisions.
 - **`<context>/docs/adr/`**: decisions scoped to a single context.
 
+**ADR numbers are unique repo-wide, not per directory.** A new ADR takes the next number across
+*every* `docs/adr/` directory in the repo, whichever one it lands in — so `docs/adr/0001-…` and
+`meta/docs/adr/0002-…` is correct and a second `0001` anywhere is not. This deliberately diverges
+from the `/domain-modeling` skill's `ADR-FORMAT.md`, which scans a single directory; follow this
+file, and note that a per-directory scan will hand you a number that is already taken. The
+directory still says whose decision it is: repo-wide, or one context's.
+
+**To get the next number, ask the check — don't search the repo:**
+
+```
+python3 meta/scripts/check_adr_numbers.py --next
+```
+
+It prints the number alone (e.g. `0003`) so it can be substituted straight into a filename, and it
+answers even while a collision exists, since that is when you most need it. The number is the
+highest in use plus one, never a gap left by a deleted ADR — reusing one would break every
+reference to the original.
+
+`meta/scripts/check_adr_numbers.py` with no arguments enforces both the uniqueness and the
+`NNNN-kebab-slug.md` filename shape, as the `ADR number uniqueness check` CI job. A duplicate is
+reported with a free number to move to, so a collision carries its own fix.
+
 If any of these files don't exist, **proceed silently**. Don't flag their absence; don't suggest
 creating them upfront. The `/domain-modeling` skill (reached via `/grill-with-docs` and
 `/improve-codebase-architecture`) creates them lazily when terms or decisions actually get resolved.
diff --git a/meta/CONTEXT.md b/meta/CONTEXT.md
new file mode 100644
index 0000000..4ff5e27
--- /dev/null
+++ b/meta/CONTEXT.md
@@ -0,0 +1,60 @@
+# Repo meta
+
+The monorepo's own automation: the repo-health checks that enforce cross-cutting invariants, and
+the shared devcontainer base image that Syndic repos build their development environments on. It
+operates on the other contexts rather than shipping anything a user runs.
+
+## Language
+
+### Devcontainer base image
+
+**Base image**:
+The devcontainer image this context assembles and publishes for Syndic repos to build on. It
+carries the container half of the plumbing and nothing else — no language toolchains, no features.
+_Avoid_: shared image, common image
+
+**Consumer**:
+A repo whose devcontainer Dockerfile `FROM`s the base image. This repo is one; `Syndic/.dotfiles`
+is the other.
+_Avoid_: client, downstream, dependent
+
+**Plumbing**:
+The bridging of host state into a container — the git common directory, the host timezone, the
+material a signed commit needs. Not git's plumbing/porcelain sense: nothing here is a low-level git
+command.
+_Avoid_: bootstrap, host integration
+
+**Host stub**:
+The `initializeCommand` script a consumer keeps, which runs on the developer's host before any
+container exists. Its job is to present host state in the shape the container consumes, without
+dictating where the host keeps anything.
+_Avoid_: initialize script, host script
+
+**Dispatcher**:
+The single command a consumer's lifecycle hooks call to apply every shared plumbing step for a
+phase. Being one command rather than a library is what lets a new step reach every consumer on its
+next base image bump, with no edit on their side.
+
+**Contract**:
+The interface between a consumer's host stub and the base image: the file names, symlink names and
+values the stub must present, plus the destinations the plumbing writes to. Conforming to it is
+what adoption means.
+
+### Repo health
+
+**Check**:
+A guard that enforces an invariant no single language toolchain owns. Whether it blocks depends on
+where it runs — CI, pre-commit, or on-save in the editor — not on what it is.
+_Avoid_: gate, guard, linter, validator
+
+**Derived file**:
+A checked-in file that is reproducible from other checked-in sources, so it is regenerated rather
+than hand-edited and its staleness is a defect a check can find. Distinct from a dependency, whose
+value comes from outside the repo and is bumped rather than derived — the same artifact can be one
+in this repo and the other in a consumer.
+_Avoid_: generated file, lockfile
+
+**Marker comment**:
+A comment that tells Renovate which version string a line carries, where the file's own syntax
+can't. A marker no pattern claims is invisible rather than broken, which is the failure mode it is
+prone to.
diff --git a/meta/docs/adr/0002-the-plumbing-contract-includes-the-host-presentation-shape.md b/meta/docs/adr/0002-the-plumbing-contract-includes-the-host-presentation-shape.md
new file mode 100644
index 0000000..3c203c2
--- /dev/null
+++ b/meta/docs/adr/0002-the-plumbing-contract-includes-the-host-presentation-shape.md
@@ -0,0 +1,42 @@
+# The plumbing contract includes the host-presentation shape
+
+The shared devcontainer base image's contract with a consumer is not merely "call the dispatcher".
+It includes the shape the host stub must present: fixed file names under `.git-plumbing/`, fixed
+symlink names beside it, and the destinations the plumbing writes to. A consumer conforms to that
+layout. What stays free is where on the host each value is read from, which is what the symlink
+indirection buys.
+
+## Considered options
+
+### The steps only — rejected
+
+Under this reading the image publishes behaviour, and each consumer presents host state however it
+likes. It is what `meta/devcontainer-base/README.md` implied by saying the stub "does not constrain
+where the host keeps anything".
+
+The code never matched it. `plumbing_apply_git_common` reads `$PLUMBING_DIR/host-git-common-path`
+by name and links it to `/host-git-common`; `plumbing_apply_all` reads `host-timezone` by name.
+Those are contract terms whether or not they are written down as such. Leaving them undocumented
+means the next consumer discovers them by breakage rather than by reading, and `Syndic/.dotfiles`
+demonstrates the cost: it presents `known_hosts` and `allowed_signers` as snapshot files where this
+repo presents them as bind-mounted symlinks, so its copy of the plumbing diverged in kind rather
+than in detail.
+
+### The steps plus the shape — accepted
+
+The contract names the files, the symlinks and the destinations, and the README documents them as
+an interface. The host stub's remaining freedom is real but bounded: it chooses what each symlink
+points at, so a developer's `known_hosts` can live anywhere, but it does not choose the name the
+container looks for.
+
+## Consequences
+
+- Adoption is conformance, not integration. `Syndic/.dotfiles` moves to the documented shape rather
+  than keeping its own; its snapshot-based handling is an artefact of age, not a variant to support.
+- A consumer that wants a different layout has one supported answer — change the symlink target —
+  and one unsupported one. That is deliberate: configurability on this axis would reintroduce the
+  divergence the shared image exists to remove.
+- The contract is versioned by the image digest like everything else, so widening it is a base
+  image change that reaches consumers on their next bump.
+- What the image cannot carry stays outside the contract and unsolved: the host stub itself, the
+  two bind mounts, and `containerEnv`. Tracked in #248.
diff --git a/meta/scripts/BUILD.bazel b/meta/scripts/BUILD.bazel
index 31b1102..aaad66d 100644
--- a/meta/scripts/BUILD.bazel
+++ b/meta/scripts/BUILD.bazel
@@ -38,6 +38,30 @@ py_test(
     ],
 )
 
+py_library(
+    name = "check_adr_numbers_lib",
+    srcs = ["check_adr_numbers.py"],
+    deps = [":_workspace_lib"],
+)
+
+py_binary(
+    name = "check_adr_numbers",
+    srcs = ["check_adr_numbers.py"],
+    main = "check_adr_numbers.py",
+    deps = [":_workspace_lib"],
+)
+
+py_test(
+    name = "test_check_adr_numbers",
+    size = "small",
+    srcs = ["test_check_adr_numbers.py"],
+    main = "test_check_adr_numbers.py",
+    deps = [
+        ":_workspace_lib",
+        ":check_adr_numbers_lib",
+    ],
+)
+
 py_library(
     name = "check_modules_lib",
     srcs = ["check_modules.py"],
diff --git a/meta/scripts/README.md b/meta/scripts/README.md
index af96aca..525cdc5 100644
--- a/meta/scripts/README.md
+++ b/meta/scripts/README.md
@@ -10,9 +10,16 @@ without blocking).
 | `check_modules.py`      | Go module matrix/config and Python workspace/lock invariants are consistent                    | `ci.yml`, `security.yml`      | —                     | `check: modules`                  |
 | `check_go_work.py`      | Every Go module in the repo is registered in `go.work`                                         | `ci.yml`                      | —                     | `check: go work`                  |
 | `check_no_cgo.py`       | No `import "C"` in our Go source and no transitive deps that compile C/C++/cgo/SWIG            | `ci.yml`                      | —                     | —                                 |
+| `check_adr_numbers.py`  | ADR numbers are unique repo-wide and filenames are `NNNN-kebab-slug.md`         | `ci.yml`                      | —                     | `check: adr numbers`              |
 | `check_secrets_dir.py`  | `secrets/` contains no committed files other than `secrets.md`                                 | `ci.yml`                      | `check-secrets-dir`   | —                                 |
 
-`_workspace.py` is a private shared helper for the four guards above (Bazel workspace discovery,
+`check_adr_numbers.py` also answers `--next`, which prints the next free ADR number and nothing
+else. That is the supported way to pick one — the alternative is a repo-wide search, since the
+numbering is global while the directories are per-context. It deliberately has no counter file to
+read: the numbers already live in the filenames, and a second copy would be a derived file needing
+its own freshness check, and a merge conflict on every concurrent ADR.
+
+`_workspace.py` is a private shared helper for the five guards above (Bazel workspace discovery,
 module enumeration). The leading underscore signals it's not a public API; `test__workspace.py`
 covers it directly.
 
diff --git a/meta/scripts/check_adr_numbers.py b/meta/scripts/check_adr_numbers.py
new file mode 100644
index 0000000..4d8cebf
--- /dev/null
+++ b/meta/scripts/check_adr_numbers.py
@@ -0,0 +1,123 @@
+#!/usr/bin/env python3
+"""
+Verifies that ADR numbers are unique across every decision-record directory in the repo:
+  - Every file in a docs/adr/ directory is named NNNN-kebab-slug.md (README.md excepted)
+  - No two ADRs anywhere in the repo share a number
+
+Numbering is repo-global, not per-directory: a context's ADR takes the next number across
+*all* of docs/adr/ and <context>/docs/adr/. See docs/agents/domain.md.
+
+Usage: ./meta/scripts/check_adr_numbers.py            # check
+       ./meta/scripts/check_adr_numbers.py --next     # print the next free number
+"""
+
+import re
+import sys
+from collections import defaultdict
+from pathlib import Path
+
+# When invoked as `python3 meta/scripts/check_adr_numbers.py` (the form used in CI and by the
+# editor task), the workspace root is not on sys.path, so `from meta.scripts.X` would fail.
+sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+
+from meta.scripts._workspace import find_files, workspace_root
+
+# Four digits so a plain sort orders them; kebab slug so filenames stay greppable and
+# case-insensitive filesystems can't collide two ADRs onto one path.
+_ADR_NAME_RE = re.compile(r"^(\d{4})-[a-z0-9]+(?:-[a-z0-9]+)*\.md$")
+
+# A directory-level readme is not a decision record. Nothing else earns an exemption:
+# an unrecognised file here is either a misnamed ADR or something in the wrong place.
+_EXEMPT = frozenset(["README.md"])
+
+
+def adr_files(root: Path) -> list[Path]:
+    """Every markdown file under a docs/adr/ directory, sorted, relative to root."""
+    return sorted(
+        p.relative_to(root)
+        for p in find_files(root, "*.md")
+        if p.parent.name == "adr" and p.parent.parent.name == "docs"
+    )
+
+
+def next_number(files: list[Path]) -> str:
+    """The lowest number no ADR has taken yet: highest in use plus one, or 0001 if none are.
+
+    Deliberately max+1 rather than count+1, so a gap left by a deleted or renamed ADR is never
+    handed out again -- a reused number would break every reference to the original.
+    """
+    used = [int(n) for n in _numbers_by_path(files)]
+    return f"{(max(used) + 1) if used else 1:04d}"
+
+
+def _numbers_by_path(files: list[Path]) -> dict[str, list[Path]]:
+    """Number -> the ADR paths claiming it. Malformed and exempt names are absent."""
+    by_number: dict[str, list[Path]] = defaultdict(list)
+    for path in files:
+        if path.name in _EXEMPT:
+            continue
+        match = _ADR_NAME_RE.match(path.name)
+        if match is not None:
+            by_number[match.group(1)].append(path)
+    return by_number
+
+
+def violations(files: list[Path]) -> list[tuple[Path, str]]:
+    """(path, message) for every malformed name and every reused number."""
+    found: list[tuple[Path, str]] = []
+    by_number = _numbers_by_path(files)
+
+    for path in files:
+        if path.name not in _EXEMPT and _ADR_NAME_RE.match(path.name) is None:
+            found.append(
+                (path, f"not a valid ADR filename: expected NNNN-kebab-slug.md, got {path.name}")
+            )
+
+    # Named in the message so a collision carries its own fix. "or later" because several
+    # duplicates all see the same free number and only the first can take it.
+    free = next_number(files)
+    for number, paths in by_number.items():
+        if len(paths) == 1:
+            continue
+        first, *rest = paths
+        for path in rest:
+            found.append(
+                (
+                    path,
+                    f"duplicate ADR number {number}: already used by {first}; "
+                    f"renumber to {free} or later",
+                )
+            )
+
+    return sorted(found)
+
+
+def run(root: Path, next_only: bool) -> int:
+    files = adr_files(root)
+
+    if next_only:
+        # A query, not a check: prints the number and nothing else, so it can be substituted
+        # into a filename. Reports a number even when the tree has violations -- `--next` is
+        # how you get out of a collision, so refusing to answer during one would be backwards.
+        print(next_number(files))
+        return 0
+
+    found = violations(files)
+
+    for path, message in found:
+        # `path:line:startCol-endCol: message`, the format the editor's problem matcher reads.
+        # The offence is the filename, so there is no interesting position inside the file.
+        print(f"{path}:1:1-2: {message}")
+
+    if not found:
+        print(f"ADR numbers are unique across {len(files)} decision record(s).")
+
+    return len(found)
+
+
+def main() -> int:
+    return run(workspace_root(), "--next" in sys.argv[1:])
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/meta/scripts/test_check_adr_numbers.py b/meta/scripts/test_check_adr_numbers.py
new file mode 100644
index 0000000..a4a2156
--- /dev/null
+++ b/meta/scripts/test_check_adr_numbers.py
@@ -0,0 +1,176 @@
+#!/usr/bin/env python3
+"""Unit tests for check_adr_numbers.py."""
+
+import contextlib
+import io
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
+
+from meta.scripts.check_adr_numbers import adr_files, next_number, run, violations
+
+
+def _paths(*names: str) -> list[Path]:
+    return [Path(n) for n in names]
+
+
+class TestAdrFiles(unittest.TestCase):
+    """Discovery has to span every docs/adr/ in the tree, and nothing else."""
+
+    def _tree(self, *rel: str) -> Path:
+        root = Path(tempfile.mkdtemp())
+        for r in rel:
+            p = root / r
+            p.parent.mkdir(parents=True, exist_ok=True)
+            p.write_text("# x\n")
+        return root
+
+    def test_finds_root_and_context_directories(self):
+        root = self._tree("docs/adr/0001-a.md", "meta/docs/adr/0002-b.md")
+        self.assertEqual(adr_files(root), _paths("docs/adr/0001-a.md", "meta/docs/adr/0002-b.md"))
+
+    def test_ignores_markdown_outside_an_adr_directory(self):
+        # A check that swept every *.md would flag the repo's ordinary docs.
+        root = self._tree("docs/adr/0001-a.md", "docs/agents/domain.md", "README.md")
+        self.assertEqual(adr_files(root), _paths("docs/adr/0001-a.md"))
+
+    def test_ignores_an_adr_directory_not_under_docs(self):
+        root = self._tree("adr/0001-a.md", "docs/adr/0002-b.md")
+        self.assertEqual(adr_files(root), _paths("docs/adr/0002-b.md"))
+
+    def test_skips_build_output_and_vcs_directories(self):
+        # find_files' skip list is what keeps a bazel-* symlink from doubling every ADR.
+        root = self._tree("docs/adr/0001-a.md", "bazel-out/docs/adr/0001-a.md")
+        self.assertEqual(adr_files(root), _paths("docs/adr/0001-a.md"))
+
+
+class TestViolations(unittest.TestCase):
+    def test_unique_numbers_pass(self):
+        self.assertEqual(violations(_paths("docs/adr/0001-a.md", "meta/docs/adr/0002-b.md")), [])
+
+    def test_duplicate_across_directories_fails(self):
+        # The case global numbering exists to prevent: per-directory numbering produces this.
+        found = violations(_paths("docs/adr/0001-a.md", "meta/docs/adr/0001-b.md"))
+        self.assertEqual(len(found), 1)
+        path, message = found[0]
+        self.assertEqual(path, Path("meta/docs/adr/0001-b.md"))
+        self.assertIn("duplicate ADR number 0001", message)
+        self.assertIn("docs/adr/0001-a.md", message)
+
+    def test_duplicate_message_names_a_free_number(self):
+        # The failure should carry its own fix rather than sending the reader back to --next.
+        found = violations(_paths("docs/adr/0001-a.md", "docs/adr/0001-b.md"))
+        self.assertIn("renumber to 0002 or later", found[0][1])
+
+    def test_duplicate_within_one_directory_fails(self):
+        found = violations(_paths("docs/adr/0001-a.md", "docs/adr/0001-b.md"))
+        self.assertEqual(len(found), 1)
+
+    def test_a_third_use_is_reported_too(self):
+        # Reporting only the first collision would let a fix land and still leave a duplicate.
+        found = violations(_paths("docs/adr/0001-a.md", "docs/adr/0001-b.md", "docs/adr/0001-c.md"))
+        self.assertEqual(len(found), 2)
+
+    def test_readme_is_exempt(self):
+        self.assertEqual(violations(_paths("docs/adr/README.md", "docs/adr/0001-a.md")), [])
+
+    def test_malformed_names_are_reported(self):
+        for name in (
+            "1-a.md",
+            "00001-a.md",
+            "0001a.md",
+            "0001-.md",
+            "0001-Mixed-Case.md",
+            "notes.md",
+        ):
+            with self.subTest(name=name):
+                found = violations(_paths(f"docs/adr/{name}"))
+                self.assertEqual(len(found), 1, f"{name} should be rejected")
+                self.assertIn("not a valid ADR filename", found[0][1])
+
+    def test_malformed_name_is_not_also_counted_as_a_number(self):
+        # A file that fails the shape has no number to collide with; reporting both would
+        # double-count one mistake.
+        found = violations(_paths("docs/adr/0001-a.md", "docs/adr/0001a.md"))
+        self.assertEqual(len(found), 1)
+        self.assertIn("not a valid ADR filename", found[0][1])
+
+    def test_valid_slugs_with_digits_are_accepted(self):
+        self.assertEqual(violations(_paths("docs/adr/0007-use-oauth2-for-sso.md")), [])
+
+
+class TestNextNumber(unittest.TestCase):
+    def test_empty_repo_starts_at_one(self):
+        self.assertEqual(next_number([]), "0001")
+
+    def test_is_max_plus_one_not_count_plus_one(self):
+        # A gap from a deleted ADR must not be handed out again: 0002 may still be cited.
+        self.assertEqual(next_number(_paths("docs/adr/0001-a.md", "docs/adr/0003-c.md")), "0004")
+
+    def test_spans_every_directory(self):
+        self.assertEqual(
+            next_number(_paths("docs/adr/0001-a.md", "meta/docs/adr/0002-b.md")), "0003"
+        )
+
+    def test_ignores_malformed_and_exempt_names(self):
+        self.assertEqual(
+            next_number(_paths("docs/adr/0001-a.md", "docs/adr/README.md", "docs/adr/9999x.md")),
+            "0002",
+        )
+
+    def test_answers_during_a_collision(self):
+        # --next is how you get out of a duplicate, so it has to work while one exists.
+        self.assertEqual(next_number(_paths("docs/adr/0001-a.md", "docs/adr/0001-b.md")), "0002")
+
+    def test_pads_to_four_digits(self):
+        self.assertEqual(next_number(_paths("docs/adr/0009-i.md")), "0010")
+
+
+class TestRun(unittest.TestCase):
+    """The CLI's two modes, driven without shelling out."""
+
+    def _tree(self, *rel: str) -> Path:
+        root = Path(tempfile.mkdtemp())
+        for r in rel:
+            p = root / r
+            p.parent.mkdir(parents=True, exist_ok=True)
+            p.write_text("# x\n")
+        return root
+
+    def _capture(self, root: Path, next_only: bool) -> tuple[int, str]:
+        buf = io.StringIO()
+        with contextlib.redirect_stdout(buf):
+            code = run(root, next_only)
+        return code, buf.getvalue()
+
+    def test_next_prints_only_the_number(self):
+        # Printed bare so it can be substituted straight into a filename.
+        root = self._tree("docs/adr/0001-a.md", "meta/docs/adr/0002-b.md")
+        code, out = self._capture(root, next_only=True)
+        self.assertEqual(code, 0)
+        self.assertEqual(out, "0003\n")
+
+    def test_next_succeeds_even_when_the_tree_is_invalid(self):
+        root = self._tree("docs/adr/0001-a.md", "docs/adr/0001-b.md")
+        code, out = self._capture(root, next_only=True)
+        self.assertEqual(code, 0)
+        self.assertEqual(out, "0002\n")
+
+    def test_check_mode_reports_and_exits_nonzero(self):
+        root = self._tree("docs/adr/0001-a.md", "meta/docs/adr/0001-b.md")
+        code, out = self._capture(root, next_only=False)
+        self.assertEqual(code, 1)
+        self.assertIn("meta/docs/adr/0001-b.md:1:1-2:", out)
+
+    def test_check_mode_is_quiet_and_zero_when_clean(self):
+        root = self._tree("docs/adr/0001-a.md", "meta/docs/adr/0002-b.md")
+        code, out = self._capture(root, next_only=False)
+        self.assertEqual(code, 0)
+        self.assertIn("unique across 2", out)
+
+
+if __name__ == "__main__":
+    unittest.main()