feat(list-handler): hand off to a fresh session after Phase 2

The merge-helper ("Let Claude handle it") system prompt now calls a new
handoff_list_handler MCP tool once every surviving task is enhanced,
instead of continuing into Phases 3-5 in the same session -- avoiding
paying for Phases 0-2's dedupe/rewrite context on every polling round of
the run/review/merge phases.

The tool broadcasts HandoffRequested; Mission Control opens a second
ConPTY tile for the SAME handler task id (no new task, HandlerBaseCommit
untouched) running a fresh handoff brief that starts at Phase 3. The
original tile stays open. Adds PromptKind.MergeHelperHandoff,
InteractiveLaunchSpecService.BuildForMergeHelperHandoffAsync, and the
GetMergeHelperHandoffLaunchSpec hub method.

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

@@ -25,6 +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;
+
     event Action? PrepStartedEvent;
     event Action<string>? PrepLineEvent;
     event Action<bool>? PrepFinishedEvent;
@@ -90,6 +94,10 @@ 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>
+    Task<LaunchSpec> GetMergeHelperHandoffLaunchSpecAsync(
+        string taskId, IReadOnlyList<string> survivingTaskIds, 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);