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/OverrideSlotService.cs

@@ -10,6 +10,7 @@ public sealed class OverrideSlotService
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
     private readonly TaskRunner _runner;
     private readonly ILogger<OverrideSlotService> _logger;
+    private readonly RunCancellationRegistry _runCancels;
 
     private readonly object _lock = new();
     private volatile QueueSlotState? _slot;
@@ -17,11 +18,13 @@ public sealed class OverrideSlotService
     public OverrideSlotService(
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
         TaskRunner runner,
-        ILogger<OverrideSlotService> logger)
+        ILogger<OverrideSlotService> logger,
+        RunCancellationRegistry runCancels)
     {
         _dbFactory = dbFactory;
         _runner = runner;
         _logger = logger;
+        _runCancels = runCancels;
     }
 
     public QueueSlotState? CurrentSlot => _slot;
@@ -66,12 +69,14 @@ public sealed class OverrideSlotService
 
             var cts = new CancellationTokenSource();
             _slot = new QueueSlotState { TaskId = taskId, StartedAt = DateTime.UtcNow, Cts = cts };
+            _runCancels.Register(taskId, cts);
 
             _ = work(cts.Token).ContinueWith(t =>
             {
                 if (t.IsFaulted)
                     _logger.LogError(t.Exception, faultMessage, taskId);
                 lock (_lock) { _slot = null; }
+                _runCancels.Unregister(taskId, cts);
                 cts.Dispose();
             }, TaskScheduler.Default);
         }