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>

tests/ClaudeDo.Worker.Tests/Hub/PlanningHubTests.cs

@@ -49,7 +49,7 @@ public sealed class PlanningHubTests : IDisposable
     private WorkerHub CreateHub()
     {
         var hub = new WorkerHub(
-            null!, null!, null!, null!, null!, null!, null!, null!,
+            null!, null!, null!, null!, null!, null!, null!, null!, null!,
             _planning, _launcher, null!, null!, null!);
         hub.Clients = new FakeHubCallerClients(_proxy);
         hub.Context = new FakeHubCallerContext();