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/Queue/QueueWaker.cs

@@ -0,0 +1,18 @@
+namespace ClaudeDo.Worker.Queue;
+
+/// <summary>
+/// Owns the wake semaphore. Producers (state mutations, hub) call Wake();
+/// the queue dispatcher awaits WaitAsync.
+/// </summary>
+public sealed class QueueWaker : IQueueWaker
+{
+    private readonly SemaphoreSlim _signal = new(0, 1);
+
+    public void Wake()
+    {
+        try { _signal.Release(); }
+        catch (SemaphoreFullException) { /* already signalled */ }
+    }
+
+    public Task WaitAsync(CancellationToken ct) => _signal.WaitAsync(ct);
+}