fix(worker): kill cancelled runs' processes and make MCP approve actually merge

- CancelAsync now signals the running Claude process of the cancelled task and
  its cascaded children via the new RunCancellationRegistry (queue + override
  slots register their CTS there) instead of only flipping DB state.
- external MCP review_task 'approve' now mirrors the hub's ApproveReview:
  unit merge for parents, ApproveAndMergeAsync for childless tasks, optional
  targetBranch; ReviewTaskResult carries mergeStatus/conflicts.

src/ClaudeDo.Worker/Queue/QueueService.cs

@@ -19,6 +19,7 @@ public sealed class QueueService : BackgroundService
     private readonly IQueuePicker _picker;
     private readonly OverrideSlotService _override;
     private readonly ITaskStateService _state;
+    private readonly RunCancellationRegistry _runCancels;
 
     private readonly object _lock = new();
     private readonly Dictionary<string, QueueSlotState> _queueSlots = new();
@@ -31,7 +32,8 @@ public sealed class QueueService : BackgroundService
         QueueWaker waker,
         IQueuePicker picker,
         OverrideSlotService overrideSlot,
-        ITaskStateService state)
+        ITaskStateService state,
+        RunCancellationRegistry runCancels)
     {
         _dbFactory = dbFactory;
         _runner = runner;
@@ -41,6 +43,7 @@ public sealed class QueueService : BackgroundService
         _picker = picker;
         _override = overrideSlot;
         _state = state;
+        _runCancels = runCancels;
     }
 
     public IReadOnlyList<(string slot, string taskId, DateTime startedAt)> GetActive()
@@ -125,12 +128,14 @@ public sealed class QueueService : BackgroundService
                     {
                         var cts = CancellationTokenSource.CreateLinkedTokenSource(stoppingToken);
                         _queueSlots[task.Id] = new QueueSlotState { TaskId = task.Id, StartedAt = DateTime.UtcNow, Cts = cts };
+                        _runCancels.Register(task.Id, cts);
 
                         _ = RunInSlotAsync(task.Id, cts.Token).ContinueWith(t =>
                         {
                             if (t.IsFaulted)
                                 _logger.LogError(t.Exception, "RunInSlotAsync failed for task {TaskId} in queue slot", task.Id);
                             lock (_lock) { _queueSlots.Remove(task.Id); }
+                            _runCancels.Unregister(task.Id, cts);
                             cts.Dispose();
                             _waker.Wake(); // Check for next task immediately.
                         }, TaskScheduler.Default);