feat(ui): wire list-handler nextPhase through the handoff hub event

HandoffRequested now carries nextPhase end to end (IWorkerClient ->
WorkerClient -> MissionControlViewModel -> GetMergeHelperHandoffLaunchSpecAsync),
and the outgoing tile is left open on handoff instead of being closed --
lookups that need the active pane for a task now use LastOrDefault since a
handler task's ConPtySessions can hold more than one pane.

src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs

@@ -25,9 +25,10 @@ public interface IWorkerClient : INotifyPropertyChanged
     /// <summary>A pending question was answered, timed out, or the run ended: (taskId, questionId).</summary>
     event Action<string, string>? TaskQuestionResolvedEvent;
 
-    /// <summary>A running list-handler session called handoff_list_handler at the end of Phase 2:
-    /// (handlerTaskId, survivingTaskIds). The UI opens a second ConPTY tile for the same task.</summary>
-    event Action<string, IReadOnlyList<string>>? HandoffRequestedEvent;
+    /// <summary>A running list-handler session called handoff_list_handler at the end of a phase:
+    /// (handlerTaskId, survivingTaskIds, nextPhase). The UI opens a second ConPTY tile for the same
+    /// task to carry out nextPhase ("wait" | "merge" | "wait_final" | "merge_final").</summary>
+    event Action<string, IReadOnlyList<string>, string>? HandoffRequestedEvent;
 
     event Action? PrepStartedEvent;
     event Action<string>? PrepLineEvent;
@@ -109,10 +110,11 @@ public interface IWorkerClient : INotifyPropertyChanged
     /// never queued) so the ConPTY tile can be task-based instead of ad-hoc. Returns the new task id.</summary>
     Task<string> CreateMergeHelperTaskAsync(
         IReadOnlyList<string> taskIds, string listId, string title, string descriptionHeader, CancellationToken ct = default);
-    /// <summary>Launch spec for the fresh ConPTY session a merge-helper run hands off to once Phase 2
-    /// is done -- reuses the SAME handler task id (no new task created).</summary>
+    /// <summary>Launch spec for the fresh ConPTY session a merge-helper run hands off to once a phase
+    /// is done -- reuses the SAME handler task id (no new task created). nextPhase picks the next
+    /// session's role: "wait" | "merge" | "wait_final" | "merge_final".</summary>
     Task<LaunchSpec> GetMergeHelperHandoffLaunchSpecAsync(
-        string taskId, IReadOnlyList<string> survivingTaskIds, CancellationToken ct = default);
+        string taskId, IReadOnlyList<string> survivingTaskIds, string nextPhase, CancellationToken ct = default);
     /// <summary>Starts a planning session and returns the launch spec for an embedded ConPTY
     /// planning terminal (replaces StartPlanningSessionAsync's external wt window).</summary>
     Task<LaunchSpec> GetPlanningStartLaunchSpecAsync(string taskId, CancellationToken ct = default);

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -49,7 +49,7 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
     public event Action<string>? TaskUpdatedEvent;
     public event Action<string, string, string>? TaskQuestionAskedEvent;
     public event Action<string, string>? TaskQuestionResolvedEvent;
-    public event Action<string, IReadOnlyList<string>>? HandoffRequestedEvent;
+    public event Action<string, IReadOnlyList<string>, string>? HandoffRequestedEvent;
     public event Action? ConnectionRestoredEvent;
     public event Action<string>? WorktreeUpdatedEvent;
     public event Action<string>? ListUpdatedEvent;
@@ -151,9 +151,9 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
             Dispatcher.UIThread.Post(() => TaskQuestionResolvedEvent?.Invoke(taskId, questionId));
         });
 
-        _hub.On<string, IReadOnlyList<string>>("HandoffRequested", (taskId, survivingTaskIds) =>
+        _hub.On<string, IReadOnlyList<string>, string>("HandoffRequested", (taskId, survivingTaskIds, nextPhase) =>
         {
-            Dispatcher.UIThread.Post(() => HandoffRequestedEvent?.Invoke(taskId, survivingTaskIds));
+            Dispatcher.UIThread.Post(() => HandoffRequestedEvent?.Invoke(taskId, survivingTaskIds, nextPhase));
         });
 
         _hub.On<string>("WorktreeUpdated", taskId =>
@@ -566,8 +566,8 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         => await _hub.InvokeAsync<string>("CreateMergeHelperTask", taskIds, listId, title, descriptionHeader, ct);
 
     public async Task<LaunchSpec> GetMergeHelperHandoffLaunchSpecAsync(
-        string taskId, IReadOnlyList<string> survivingTaskIds, CancellationToken ct = default)
-        => await _hub.InvokeAsync<LaunchSpec>("GetMergeHelperHandoffLaunchSpec", taskId, survivingTaskIds, ct);
+        string taskId, IReadOnlyList<string> survivingTaskIds, string nextPhase, CancellationToken ct = default)
+        => await _hub.InvokeAsync<LaunchSpec>("GetMergeHelperHandoffLaunchSpec", taskId, survivingTaskIds, nextPhase, ct);
 
     public async Task<LaunchSpec> GetPlanningStartLaunchSpecAsync(string taskId, CancellationToken ct = default)
         => await _hub.InvokeAsync<LaunchSpec>("GetPlanningStartLaunchSpec", taskId, ct);