[gh-aw] Add nightly-fix-finder agentic workflow

.github/skills/android-reviewer/references/msbuild-rules.md

@@ -16,7 +16,7 @@ causes broken builds for every .NET Android developer.
 | **Use `AsyncTask` for background work** | Tasks that need `async`/`await` should extend `AsyncTask` and override `RunTaskAsync()`. It handles `Yield()`, `try`/`finally`, and `Reacquire()` automatically. Use `AsyncTask.Log*` helpers for logging from the background thread — calling `Log.LogMessage` directly can cause IDE hangs. Use full paths on background threads (`Environment.CurrentDirectory` may differ if the task is on another MSBuild node). Leverage the `WhenAll` extension for parallel work over `ITaskItem[]`. |
 | **Return `!Log.HasLoggedErrors`** | `RunTask()` must return `!Log.HasLoggedErrors`. Do not return `true`/`false` directly — it skips the centralized error-tracking mechanism. |
 | **Use `Log.LogCoded*` methods** | Errors and warnings must use `Log.LogCodedError("XA####", …)` or `Log.LogCodedWarning("XA####", …)` — never bare `Log.LogError` without a code. Error messages should come from `Properties.Resources`. |
-| **XA error codes** | Error codes follow `XA####` (4+ digits). New codes must not collide with existing ones. Check `Properties/Resources.cs.resx` for used codes. Every new code must have a documentation entry and be localized. (Postmortem `#10`) |
+| **XA error codes** | Error codes follow `XA####` (4+ digits). New codes must not collide with existing ones. Check `Properties/Resources.cs.resx` for used codes. Every new code must have a corresponding markdown documentation file at `Documentation/docs-mobile/messages/xa####.md` (following the existing format with frontmatter, example messages, issue explanation, and solution), and the code must be added to the table of contents in `Documentation/docs-mobile/messages/index.md`. (Postmortem `#10`) |
 | **`[Required]` properties** | `[Required]` properties must be non-nullable with a default: `public string Foo { get; set; } = "";` or `public ITaskItem[] Bar { get; set; } = [];`. Non-`[Required]` and `[Output]` properties must be nullable (`string?`, `ITaskItem[]?`). Mark properties `[Required]` when the task crashes without them. (Postmortem `#51`) |
 | **`UsingTask` for internal tasks** | `<UsingTask/>` elements for `xa-prep-tasks` and `BootstrapTasks` (internal, not shipped) must use `TaskFactory="TaskHostFactory"` and `Runtime="NET"`. Do NOT add these attributes to shipped task definitions in `Xamarin.Android.Common.targets` or `Microsoft.Android.Sdk/*.targets`. |
 | **Caching with `RegisterTaskObject`** | Use `BuildEngine4.RegisterTaskObject()` (via the `RegisterTaskObjectAssemblyLocal()` extension method) instead of `static` variables for sharing data between tasks or across builds. Use `as` for casts to avoid `InvalidCastException`. Cache keys should include context that invalidates properly (device target, file path, version). Cache primitive/small values only. |