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/External/ExternalMcpServiceTests.cs

@@ -115,7 +115,9 @@ public sealed class ExternalMcpServiceTests : IDisposable
         var argsBuilder = new ClaudeArgsBuilder();
         var runner = new TaskRunner(fake, dbFactory, broadcaster, wtManager, argsBuilder, cfg,
             NullLogger<TaskRunner>.Instance, TaskStateServiceBuilder.Build(dbFactory).State);
-        return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance);
+        var waker = new ClaudeDo.Worker.Queue.QueueWaker();
+        var picker = new ClaudeDo.Worker.Queue.QueuePicker(dbFactory);
+        return new QueueService(dbFactory, runner, cfg, NullLogger<QueueService>.Instance, waker, picker);
     }
 
     [Fact]