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/Program.cs

@@ -92,6 +92,7 @@ builder.Services.AddSingleton<PlanningChainCoordinator>();
 builder.Services.AddSingleton<QueueWaker>();
 builder.Services.AddSingleton<IQueueWaker>(sp => sp.GetRequiredService<QueueWaker>());
 builder.Services.AddSingleton<IQueuePicker, QueuePicker>();
+builder.Services.AddSingleton<RunCancellationRegistry>();
 
 builder.Services.AddSingleton<Func<ITaskStateService>>(sp => () => sp.GetRequiredService<ITaskStateService>());
 builder.Services.AddSingleton<ITaskStateService>(sp => new TaskStateService(
@@ -99,6 +100,7 @@ builder.Services.AddSingleton<ITaskStateService>(sp => new TaskStateService(
     sp.GetRequiredService<HubBroadcaster>(),
     sp.GetRequiredService<IQueueWaker>(),
     sp.GetRequiredService<PlanningChainCoordinator>(),
+    sp.GetRequiredService<RunCancellationRegistry>(),
     sp.GetRequiredService<ILogger<TaskStateService>>()));
 
 // Agent file management.
@@ -269,6 +271,7 @@ if (cfg.ExternalMcpPort > 0)
     externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<GitService>());
     externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<WorktreeMaintenanceService>());
     externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<TaskMergeService>());
+    externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<PlanningMergeOrchestrator>());
     externalBuilder.Services.AddScoped<ExternalMcpService>();
     externalBuilder.Services.AddScoped<BatchMcpTools>();
     externalBuilder.Services.AddScoped<ListMcpTools>();