refactor(worker/queue): split queue waker and picker, auto-wake on enqueue

Slice 3 of the worker state and queue consolidation refactor.

- Add IQueueWaker / QueueWaker (singleton holding the wake semaphore).
- Add IQueuePicker / QueuePicker; raw SQL UPDATE...RETURNING moves out of
  TaskRepository.GetNextQueuedAgentTaskAsync (deleted) and now also filters
  on blocked_by_task_id IS NULL and writes started_at on claim.
- TaskStateService takes IQueueWaker directly; the Func<QueueService>
  indirection is gone. State transitions to Queued auto-wake the dispatcher.
- QueueService waits via the shared waker and dispatches via the picker.
- Drop explicit _queue.WakeQueue() calls in WorkerHub.QueuePlanningSubtasksAsync
  and ExternalMcpService.AddTask. The hub WakeQueue endpoint stays for
  diagnostics, delegating to _waker.Wake().
- Migrate tests; pre-existing flaky AppSettings/ExternalMcp tests untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -117,7 +117,7 @@ public sealed class ExternalMcpService
             ListId = listId,
             Title = title,
             Description = description,
-            Status = queueImmediately ? TaskStatus.Queued : TaskStatus.Manual,
+            Status = TaskStatus.Manual,
             CreatedAt = DateTime.UtcNow,
             CommitType = list.DefaultCommitType,
             CreatedBy = createdBy,
@@ -128,7 +128,13 @@ public sealed class ExternalMcpService
             await _tasks.SetTagsAsync(entity.Id, tags, cancellationToken);
 
         if (queueImmediately)
-            _queue.WakeQueue();
+        {
+            // Routes through TaskStateService so the queue is woken automatically.
+            var enqueue = await _state.EnqueueAsync(entity.Id, cancellationToken);
+            if (!enqueue.Ok)
+                throw new InvalidOperationException(enqueue.Reason ?? "Cannot enqueue task.");
+            entity.Status = TaskStatus.Queued;
+        }
 
         await _broadcaster.TaskUpdated(entity.Id);
         return ToDto(entity);