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.

tests/ClaudeDo.Worker.Tests/Infrastructure/TaskStateServiceBuilder.cs

@@ -17,13 +17,15 @@ public static class TaskStateServiceBuilder
         PlanningChainCoordinator Chain,
         CapturingHubContext Hub,
         Func<int> WakeCount,
-        CountingQueueWaker Waker);
+        CountingQueueWaker Waker,
+        RunCancellationRegistry RunCancels);
 
     public static Built Build(IDbContextFactory<ClaudeDoDbContext> dbFactory)
     {
         var hub = new CapturingHubContext();
         var broadcaster = new HubBroadcaster(hub);
         var waker = new CountingQueueWaker();
+        var runCancels = new RunCancellationRegistry();
 
         TaskStateService? state = null;
         var chain = new PlanningChainCoordinator(dbFactory, () => state!);
@@ -32,9 +34,10 @@ public static class TaskStateServiceBuilder
             broadcaster,
             waker,
             chain,
+            runCancels,
             NullLogger<TaskStateService>.Instance);
 
-        return new Built(state, chain, hub, () => waker.Count, waker);
+        return new Built(state, chain, hub, () => waker.Count, waker, runCancels);
     }
 }