feat(ui): add merge-target dropdown and merge-all controls to planning detail

- Add SubtaskDiffDto and CombinedDiffResultDto to PlanningDtos.cs
- Extend IWorkerClient with 5 planning merge methods and 5 events
- Implement methods and hub subscriptions on WorkerClient
- Add Status and WorktreeState to SubtaskRowViewModel
- Add MergeTargetBranches, SelectedMergeTarget, CanMergeAll,
  MergeAllDisabledReason, MergeAllError, RecomputeCanMergeAll,
  MergeAllCommand, ReviewCombinedDiffCommand (Task 14 TODO)
  to DetailsIslandViewModel
- Add planning merge section to DetailsIslandView.axaml
  (merge target ComboBox + buttons + error label), gated on
  Task.IsPlanningParent
- Add 4 xUnit tests covering CanMergeAll logic and DTO shape

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -49,6 +49,12 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
     public event Action<string>? ListUpdatedEvent;
     public event Action<WorkerLogEntry>? WorkerLogReceivedEvent;
 
+    public event Action<string, string>? PlanningMergeStartedEvent;
+    public event Action<string, string>? PlanningSubtaskMergedEvent;
+    public event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
+    public event Action<string>? PlanningMergeAbortedEvent;
+    public event Action<string>? PlanningCompletedEvent;
+
     public WorkerClient(string signalRUrl)
     {
         _hub = new HubConnectionBuilder()
@@ -123,6 +129,31 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         {
             WorkerLogReceivedEvent?.Invoke(new WorkerLogEntry(message, level, timestampUtc));
         });
+
+        _hub.On<string, string>("PlanningMergeStarted", (planningTaskId, targetBranch) =>
+        {
+            Dispatcher.UIThread.Post(() => PlanningMergeStartedEvent?.Invoke(planningTaskId, targetBranch));
+        });
+
+        _hub.On<string, string>("PlanningSubtaskMerged", (planningTaskId, subtaskId) =>
+        {
+            Dispatcher.UIThread.Post(() => PlanningSubtaskMergedEvent?.Invoke(planningTaskId, subtaskId));
+        });
+
+        _hub.On<string, string, IReadOnlyList<string>>("PlanningMergeConflict", (planningTaskId, subtaskId, conflictedFiles) =>
+        {
+            Dispatcher.UIThread.Post(() => PlanningMergeConflictEvent?.Invoke(planningTaskId, subtaskId, conflictedFiles));
+        });
+
+        _hub.On<string>("PlanningMergeAborted", planningTaskId =>
+        {
+            Dispatcher.UIThread.Post(() => PlanningMergeAbortedEvent?.Invoke(planningTaskId));
+        });
+
+        _hub.On<string>("PlanningCompleted", planningTaskId =>
+        {
+            Dispatcher.UIThread.Post(() => PlanningCompletedEvent?.Invoke(planningTaskId));
+        });
     }
 
     public Task StartAsync()
@@ -362,6 +393,46 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
     public async Task<int> GetPendingDraftCountAsync(string taskId, CancellationToken ct = default)
         => await _hub.InvokeAsync<int>("GetPendingDraftCountAsync", taskId, ct);
 
+    public async Task<IReadOnlyList<SubtaskDiffDto>> GetPlanningAggregateAsync(string planningTaskId)
+    {
+        try
+        {
+            var result = await _hub.InvokeAsync<List<SubtaskDiffDto>>("GetPlanningAggregate", planningTaskId);
+            return result ?? [];
+        }
+        catch
+        {
+            return [];
+        }
+    }
+
+    public async Task<CombinedDiffResultDto?> BuildPlanningIntegrationBranchAsync(string planningTaskId, string targetBranch)
+    {
+        try
+        {
+            return await _hub.InvokeAsync<CombinedDiffResultDto>("BuildPlanningIntegrationBranch", planningTaskId, targetBranch);
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    public async Task MergeAllPlanningAsync(string planningTaskId, string targetBranch)
+    {
+        await _hub.InvokeAsync("MergeAllPlanning", planningTaskId, targetBranch);
+    }
+
+    public async Task ContinuePlanningMergeAsync(string planningTaskId)
+    {
+        await _hub.InvokeAsync("ContinuePlanningMerge", planningTaskId);
+    }
+
+    public async Task AbortPlanningMergeAsync(string planningTaskId)
+    {
+        await _hub.InvokeAsync("AbortPlanningMerge", planningTaskId);
+    }
+
     // IWorkerClient explicit implementations (drop typed return values)
     async Task IWorkerClient.StartPlanningSessionAsync(string taskId, CancellationToken ct)
         => await StartPlanningSessionAsync(taskId, ct);