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.Worker/Runner/Interfaces/IInteractiveLaunchSpecService.cs

@@ -42,4 +42,12 @@ public interface IInteractiveLaunchSpecService
     /// is empty or the list has no existing working directory.</summary>
     Task<string> CreateMergeHelperTaskAsync(
         IReadOnlyList<string> taskIds, string listId, string title, string descriptionHeader, CancellationToken ct);
+
+    /// <summary>Builds a LaunchSpec for the fresh ConPTY session a merge-helper run hands off to
+    /// once Phase 2 (enhance) is done -- reuses the SAME handler task id (no new task, HandlerBaseCommit
+    /// untouched) and the unmodified merge-helper system prompt, writing only a fresh handoff kickoff
+    /// naming the surviving tasks. Throws KeyNotFoundException if the task/list doesn't exist;
+    /// InvalidOperationException if survivingTaskIds is empty or the list has no working directory.</summary>
+    Task<LaunchSpec> BuildForMergeHelperHandoffAsync(
+        string taskId, IReadOnlyList<string> survivingTaskIds, CancellationToken ct);
 }