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/State/TaskStateService.cs

@@ -15,6 +15,7 @@ public sealed class TaskStateService : ITaskStateService
     private readonly HubBroadcaster _broadcaster;
     private readonly IQueueWaker _waker;
     private readonly PlanningChainCoordinator _chain;
+    private readonly RunCancellationRegistry _runCancels;
     private readonly ILogger<TaskStateService> _logger;
 
     public TaskStateService(
@@ -22,12 +23,14 @@ public sealed class TaskStateService : ITaskStateService
         HubBroadcaster broadcaster,
         IQueueWaker waker,
         PlanningChainCoordinator chain,
+        RunCancellationRegistry runCancels,
         ILogger<TaskStateService> logger)
     {
         _dbFactory = dbFactory;
         _broadcaster = broadcaster;
         _waker = waker;
         _chain = chain;
+        _runCancels = runCancels;
         _logger = logger;
     }
 
@@ -255,6 +258,13 @@ public sealed class TaskStateService : ITaskStateService
                         .SetProperty(t => t.BlockedByTaskId, (string?)null), ct);
         }
 
+        // Also stop the actual Claude processes: the DB flip above doesn't reach a run
+        // already executing in a queue/override slot — without this it would keep going
+        // and only fail its final (now-invalid) transition.
+        _runCancels.TryCancel(taskId);
+        foreach (var childId in cancelledChildIds)
+            _runCancels.TryCancel(childId);
+
         await OnChildTerminalAsync(taskId, TaskStatus.Cancelled);
         await _broadcaster.TaskUpdated(taskId);
         foreach (var childId in cancelledChildIds)