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/RunCancellationRegistry.cs

@@ -0,0 +1,35 @@
+using System.Collections.Concurrent;
+
+namespace ClaudeDo.Worker.Queue;
+
+/// Maps a running task id to the CancellationTokenSource driving its Claude process.
+/// QueueService and OverrideSlotService register their slots here so components that
+/// cancel tasks DB-side (TaskStateService.CancelAsync and its child cascade) can also
+/// stop the process — without depending on the queue services, which would create a
+/// DI cycle (QueueService → TaskRunner → ITaskStateService).
+public sealed class RunCancellationRegistry
+{
+    private readonly ConcurrentDictionary<string, CancellationTokenSource> _running = new(StringComparer.Ordinal);
+
+    public void Register(string taskId, CancellationTokenSource cts) => _running[taskId] = cts;
+
+    /// Removes the registration only if <paramref name="cts"/> is still the registered
+    /// one — a re-run may already have registered a newer CTS under the same task id.
+    public void Unregister(string taskId, CancellationTokenSource cts) =>
+        _running.TryRemove(new KeyValuePair<string, CancellationTokenSource>(taskId, cts));
+
+    public bool TryCancel(string taskId)
+    {
+        if (!_running.TryGetValue(taskId, out var cts)) return false;
+        try
+        {
+            cts.Cancel();
+            return true;
+        }
+        catch (ObjectDisposedException)
+        {
+            // Slot cleanup raced us; the run is already finished.
+            return false;
+        }
+    }
+}