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/Hub/HubBroadcaster.cs

@@ -32,6 +32,11 @@ public sealed class HubBroadcaster : IPrimeBroadcaster, IRefineBroadcaster
     public Task TaskQuestionResolved(string taskId, string questionId) =>
         _hub.Clients.All.SendAsync("TaskQuestionResolved", taskId, questionId);
 
+    // A running list-handler session called handoff_list_handler at the end of Phase 2 -- the UI
+    // opens a second ConPTY tile for the same handler task id to carry out Phases 3-5.
+    public Task HandoffRequested(string taskId, IReadOnlyList<string> survivingTaskIds) =>
+        _hub.Clients.All.SendAsync("HandoffRequested", taskId, survivingTaskIds);
+
     public Task ListUpdated(string listId) =>
         _hub.Clients.All.SendAsync("ListUpdated", listId);