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/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))