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);
+}