[docs] cli: document channel persistence in aspire.config.json for init and update

src/frontend/src/content/docs/reference/cli/commands/aspire-init.mdx

@@ -42,6 +42,42 @@ When a `.sln` or `.slnx` is present, the C# option creates a **project-based App
 
 In all cases, the AppHost language does not limit what you can orchestrate — C#, JavaScript, Python, Go, Rust, Java, containers, and more can all be modeled in the same application model.
 
+### Channel persistence in aspire.config.json
+
+When `aspire init` runs with an explicit, non-stable channel (such as `daily` or `staging`), it writes a top-level `channel` key into the generated `aspire.config.json`. This ensures that subsequent commands — `aspire add`, `aspire update`, and others — resolve packages and templates against the same channel the project was initialized with, rather than falling back to the implicit stable default.
+
+For example, running a daily CLI build produces:
+
+```json title="aspire.config.json" {5}
+{
+  "appHost": {
+    "path": "apphost.cs"
+  },
+  "channel": "daily",
+  "profiles": {
+    "https": {
+      "applicationUrl": "https://localhost:17055;http://localhost:15013",
+      "environmentVariables": {
+        "ASPIRE_DASHBOARD_OTLP_GRPC_ENDPOINT_URL": "https://localhost:16168",
+        "ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL": "https://localhost:22043"
+      }
+    },
+    "http": {
+      "applicationUrl": "http://localhost:15013",
+      "environmentVariables": {
+        "ASPIRE_DASHBOARD_OTLP_GRPC_ENDPOINT_URL": "http://localhost:19147",
+        "ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL": "http://localhost:20216",
+        "ASPIRE_ALLOW_UNSECURED_TRANSPORT": "true"
+      }
+    }
+  }
+}
+```
+
+If `aspire.config.json` already contains a `channel` value (for example, because you edited it manually or migrated from an older project), the value is preserved and not overwritten.
+
+Only channels that are registered as _explicit_ in the packaging service are persisted, with one exception: the `stable` channel is treated as the portable default and is intentionally not pinned.
+
 ### The aspireify skill
 
 After the skeleton is dropped, `aspire init` has an option to install the `aspireify` skill into your AI agent's skill directory (the same directory used by `aspire agent init`). The skill is a Markdown file with instructions that guide your AI coding agent through completing the Aspire wiring for your specific codebase.

src/frontend/src/content/docs/reference/cli/commands/aspire-update.mdx

@@ -50,6 +50,8 @@ The following options are available:
 
   Channel to update to (`stable`, `staging`, `daily`).
 
+  When you switch to a persistable channel, such as `daily`, `aspire update` also rewrites the top-level `channel` key in `aspire.config.json` when the file already exists. Only channels registered as _explicit_ channels are persisted, with one exception: the `stable` channel is treated as the portable default and is intentionally not pinned. The persisted channel keeps subsequent `aspire add` and `aspire update` runs resolving packages against the channel you just switched to.
+
 - **`-y, --yes`**
 
   Automatically confirm all prompts without user input. Use this option in CI/CD pipelines or other non-interactive environments to skip confirmation dialogs.