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/WorkerHub.cs

@@ -751,6 +751,16 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         return taskId;
     });
 
+    // Builds the launch spec 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 created), only a
+    // new session dir + handoff kickoff naming the surviving tasks.
+    public Task<LaunchSpec> GetMergeHelperHandoffLaunchSpec(string taskId, string[] survivingTaskIds) => HubGuard(() =>
+    {
+        if (_interactiveLaunchSpec is null)
+            throw new InvalidOperationException("Interactive launch spec service is not configured.");
+        return _interactiveLaunchSpec.BuildForMergeHelperHandoffAsync(taskId, survivingTaskIds, Context.ConnectionAborted);
+    });
+
     // Starts a planning session (worktree + prompt files + token, task -> Planning) and returns
     // the launch spec for an embedded ConPTY planning terminal -- the ConPTY replacement for
     // StartPlanningSessionAsync's external wt window. On any spec-build failure the just-started