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/Services/QueueService.cs

@@ -2,6 +2,7 @@ using ClaudeDo.Data;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
 using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Queue;
 using ClaudeDo.Worker.Runner;
 using Microsoft.EntityFrameworkCore;
 
@@ -20,23 +21,27 @@ public sealed class QueueService : BackgroundService
     private readonly TaskRunner _runner;
     private readonly WorkerConfig _cfg;
     private readonly ILogger<QueueService> _logger;
+    private readonly QueueWaker _waker;
+    private readonly IQueuePicker _picker;
 
     private readonly object _lock = new();
     private volatile QueueSlotState? _queueSlot;
     private volatile QueueSlotState? _overrideSlot;
 
-    private readonly SemaphoreSlim _wakeSignal = new(0, 1);
-
     public QueueService(
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
         TaskRunner runner,
         WorkerConfig cfg,
-        ILogger<QueueService> logger)
+        ILogger<QueueService> logger,
+        QueueWaker waker,
+        IQueuePicker picker)
     {
         _dbFactory = dbFactory;
         _runner = runner;
         _cfg = cfg;
         _logger = logger;
+        _waker = waker;
+        _picker = picker;
     }
 
     public IReadOnlyList<(string slot, string taskId, DateTime startedAt)> GetActive()
@@ -49,13 +54,6 @@ public sealed class QueueService : BackgroundService
         return list;
     }
 
-    public void WakeQueue()
-    {
-        // Release if not already signalled.
-        try { _wakeSignal.Release(); }
-        catch (SemaphoreFullException) { /* already signalled */ }
-    }
-
     public async Task RunNow(string taskId)
     {
         using (var context = _dbFactory.CreateDbContext())
@@ -147,25 +145,14 @@ public sealed class QueueService : BackgroundService
             try
             {
                 // Wait for wake signal or backstop timer.
-                var wakeTask = _wakeSignal.WaitAsync(stoppingToken);
+                var wakeTask = _waker.WaitAsync(stoppingToken);
                 var timerTask = timer.WaitForNextTickAsync(stoppingToken).AsTask();
 
                 await Task.WhenAny(wakeTask, timerTask);
 
-                // Drain wake signal if it fired.
-                if (wakeTask.IsCompletedSuccessfully)
-                {
-                    // Good — signal consumed.
-                }
-
                 if (_queueSlot is not null) continue;
 
-                TaskEntity? task;
-                using (var context = _dbFactory.CreateDbContext())
-                {
-                    var taskRepo = new TaskRepository(context);
-                    task = await taskRepo.GetNextQueuedAgentTaskAsync(DateTime.UtcNow, stoppingToken);
-                }
+                var task = await _picker.ClaimNextAsync(DateTime.UtcNow, stoppingToken);
                 if (task is null) continue;
 
                 lock (_lock)
@@ -181,7 +168,7 @@ public sealed class QueueService : BackgroundService
                             _logger.LogError(t.Exception, "RunInSlotAsync failed for task {TaskId} in queue slot", task.Id);
                         lock (_lock) { _queueSlot = null; }
                         cts.Dispose();
-                        WakeQueue(); // Check for next task immediately.
+                        _waker.Wake(); // Check for next task immediately.
                     }, TaskScheduler.Default);
                 }
             }