fix(worker): document and test Queued→Failed guard in FailAsync

OverrideSlotService dispatches RunAsync before calling StartRunningAsync,
so a preflight failure (list not found, worktree setup) can reach MarkFailed
while the task is still Queued. The guard is intentional, not dead code.

- Add comment in FailAsync explaining the OverrideSlotService preflight gap
- Add FailAsync_FromQueued_TransitionsToFailed test
- Update CLAUDE.md transition table with the precise rationale

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/ClaudeDo.Worker.Tests/State/TaskStateServiceTests.cs

@@ -232,6 +232,21 @@ public sealed class TaskStateServiceTests : IDisposable
         Assert.Equal("boom", t.Result);
     }
 
+    [Fact]
+    public async Task FailAsync_FromQueued_TransitionsToFailed()
+    {
+        // OverrideSlotService can call MarkFailed before StartRunningAsync when a
+        // preflight step (list lookup, worktree setup) fails — the task is still Queued.
+        var id = await SeedTaskAsync(TaskStatus.Queued);
+
+        var result = await _sut.FailAsync(id, DateTime.UtcNow, "list not found", default);
+
+        Assert.True(result.Ok);
+        var t = await GetTaskAsync(id);
+        Assert.Equal(TaskStatus.Failed, t.Status);
+        Assert.Equal("list not found", t.Result);
+    }
+
     [Fact]
     public async Task FailAsync_FromDone_Rejects()
     {