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>

src/ClaudeDo.Worker/CLAUDE.md

@@ -69,7 +69,7 @@ Allowed transitions (enforced by `TaskStateService`):
 
 ```
 Idle               → Queued | Running (RunNow)
-Queued             → Running | Cancelled | Idle | Failed (runner guard)
+Queued             → Running | Cancelled | Idle | Failed (OverrideSlotService preflight gap: RunAsync can fail before StartRunningAsync is called)
 Running            → WaitingForReview (standalone success, no children)
                    | WaitingForChildren (parent with pending children)
                    | Done (planning/improvement child success) | Failed | Cancelled

src/ClaudeDo.Worker/State/TaskStateService.cs

@@ -198,6 +198,9 @@ public sealed class TaskStateService : ITaskStateService
     {
         await using (var ctx = await _dbFactory.CreateDbContextAsync(ct))
         {
+            // Queued is intentional: OverrideSlotService dispatches RunAsync before calling
+            // StartRunningAsync, so a preflight failure (list not found, worktree setup) can
+            // reach MarkFailed while the task is still Queued in the DB.
             var affected = await ctx.Tasks
                 .Where(t => t.Id == taskId &&
                     (t.Status == TaskStatus.Running || t.Status == TaskStatus.Queued))

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()
     {