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/Services/QueueServiceSlotGuardTests.cs

@@ -3,6 +3,7 @@ using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
 using ClaudeDo.Worker.Config;
 using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Queue;
 using ClaudeDo.Worker.Runner;
 using ClaudeDo.Worker.Services;
 using ClaudeDo.Worker.Tests.Infrastructure;
@@ -45,6 +46,8 @@ public sealed class QueueServiceSlotGuardTests : IDisposable
         try { Directory.Delete(_tempDir, true); } catch { }
     }
 
+    private QueueWaker _waker = null!;
+
     private (QueueService service, FakeClaudeProcess fakeProcess) CreateService(
         Func<string, string, string, Func<string, Task>, CancellationToken, Task<RunResult>>? handler = null)
     {
@@ -55,7 +58,9 @@ public sealed class QueueServiceSlotGuardTests : IDisposable
         var argsBuilder = new ClaudeArgsBuilder();
         var runner = new TaskRunner(fake, dbFactory, broadcaster, wtManager, argsBuilder, _cfg,
             NullLogger<TaskRunner>.Instance, TaskStateServiceBuilder.Build(dbFactory).State);
-        var service = new QueueService(dbFactory, runner, _cfg, NullLogger<QueueService>.Instance);
+        _waker = new QueueWaker();
+        var picker = new QueuePicker(dbFactory);
+        var service = new QueueService(dbFactory, runner, _cfg, NullLogger<QueueService>.Instance, _waker, picker);
         return (service, fake);
     }
 
@@ -102,7 +107,7 @@ public sealed class QueueServiceSlotGuardTests : IDisposable
 
         using var cts = new CancellationTokenSource();
         await service.StartAsync(cts.Token);
-        service.WakeQueue();
+        _waker.Wake();
 
         // Wait until the queue slot has actually picked up the task.
         await queuePickedUp.Task.WaitAsync(TimeSpan.FromSeconds(5));
@@ -132,7 +137,7 @@ public sealed class QueueServiceSlotGuardTests : IDisposable
 
         using var cts = new CancellationTokenSource();
         await service.StartAsync(cts.Token);
-        service.WakeQueue();
+        _waker.Wake();
 
         await queuePickedUp.Task.WaitAsync(TimeSpan.FromSeconds(5));