feat(ui): planning commands and expand/collapse in TasksIslandViewModel

- Add IWorkerClient interface; WorkerClient implements it
- TasksIslandViewModel accepts IWorkerClient? and gains OpenPlanningSession,
  ResumePlanningSession, DiscardPlanningSession, FinalizePlanningSession,
  and ToggleExpand commands
- Regroup() is hierarchy-aware: children of collapsed planning parents are hidden
- InternalsVisibleTo ClaudeDo.Worker.Tests for Regroup()
- 4 new unit tests covering collapse/expand and guard logic

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

src/ClaudeDo.Ui/Services/IWorkerClient.cs

@@ -0,0 +1,10 @@
+namespace ClaudeDo.Ui.Services;
+
+public interface IWorkerClient
+{
+    Task WakeQueueAsync();
+    Task StartPlanningSessionAsync(string taskId, CancellationToken ct = default);
+    Task ResumePlanningSessionAsync(string taskId, CancellationToken ct = default);
+    Task DiscardPlanningSessionAsync(string taskId, CancellationToken ct = default);
+    Task FinalizePlanningSessionAsync(string taskId, bool queueAgentTasks = true, CancellationToken ct = default);
+}

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -25,7 +25,7 @@ sealed class IndefiniteRetryPolicy : IRetryPolicy
         _delays[Math.Min(retryContext.PreviousRetryCount, _delays.Length - 1)];
 }
 
-public partial class WorkerClient : ObservableObject, IAsyncDisposable
+public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerClient
 {
     private readonly HubConnection _hub;
     private CancellationTokenSource? _startCts;
@@ -362,6 +362,16 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable
     public async Task<int> GetPendingDraftCountAsync(string taskId, CancellationToken ct = default)
         => await _hub.InvokeAsync<int>("GetPendingDraftCountAsync", taskId, ct);
 
+    // IWorkerClient explicit implementations (drop typed return values)
+    async Task IWorkerClient.StartPlanningSessionAsync(string taskId, CancellationToken ct)
+        => await StartPlanningSessionAsync(taskId, ct);
+    async Task IWorkerClient.ResumePlanningSessionAsync(string taskId, CancellationToken ct)
+        => await ResumePlanningSessionAsync(taskId, ct);
+    async Task IWorkerClient.DiscardPlanningSessionAsync(string taskId, CancellationToken ct)
+        => await DiscardPlanningSessionAsync(taskId, ct);
+    async Task IWorkerClient.FinalizePlanningSessionAsync(string taskId, bool queueAgentTasks, CancellationToken ct)
+        => await FinalizePlanningSessionAsync(taskId, queueAgentTasks, ct);
+
     // DTOs for deserializing hub responses
     private sealed class ActiveTaskDto
     {