docs: add session learnings to CLAUDE.md

Add dependency management gotchas, CI workflow notes, code pattern
reminders, and git tips discovered during comprehensive codebase audit.

Signed-off-by: UncleSp1d3r <unclesp1d3r@evilbitlabs.io>

Author: unclesp1d3r

CLAUDE.md

@@ -1 +1,27 @@
 @AGENTS.md
+
+## Session Learnings
+
+### Dependency Management
+
+- Cargo.toml may be reformatted by `cargo-sort` pre-commit hook — run `just ci-check` twice if it fails on "Cargo.toml is sorted"
+- Never bump `rand` or `rand_chacha` without updating all generator code — rand 0.9→0.10 is a breaking API change
+- Always verify the original code compiles (`git stash && cargo check`) before "fixing" reported build issues
+- Cargo.lock is checked in — restore it with `git checkout HEAD -- Cargo.lock` if dep resolution changes unexpectedly
+
+### CI Workflow
+
+- `just ci-check` is the mandatory gate before commits — includes pre-commit hooks, clippy, tests, coverage, and dist-plan
+- `dist-plan` may fail due to `cargo-dist` version mismatch — this is an environment issue, not a code issue
+- Pre-commit hooks auto-fix files (cargo-sort, fmt) — a second run should pass after auto-fixes
+
+### Code Patterns
+
+- Error message strings are asserted in both unit tests (`src/`) and integration tests (`tests/`) — changing an error message requires updating both
+- `VlanConfig` fields are `pub` — mutations bypass `new()` validation (known tech debt, tracked in issue #105)
+- XML template path (`xml/template.rs`) is the production code path; `xml/{builder,engine,injection,generator}.rs` are unused
+
+### Git
+
+- Default push may try all tracking branches — `git push origin <branch>` to push only the feature branch
+- DCO sign-off required: `git commit -s`