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/ViewModels/MissionControlViewModel.cs

@@ -20,6 +20,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
     private readonly Action<string, string, string, DateTime> _onTaskFinished;
     private readonly Action<string> _onTaskUpdated;
     private readonly Action _onConnectionRestored;
+    private readonly Action<string, IReadOnlyList<string>> _onHandoffRequested;
 
     // Embedded ConPTY sessions (task-based only) — a manual cockpit detached from the
     // review/merge/status machinery.
@@ -77,6 +78,9 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         _onConnectionRestored = () => { _ = RefreshQueueAsync(); };
         _worker.ConnectionRestoredEvent += _onConnectionRestored;
 
+        _onHandoffRequested = (taskId, survivingTaskIds) => { _ = OpenMergeHelperHandoffConPtySessionAsync(taskId, survivingTaskIds); };
+        _worker.HandoffRequestedEvent += _onHandoffRequested;
+
         _ = RefreshQueueAsync();
     }
 
@@ -243,6 +247,34 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
             () => DescribeAsync(() => _worker.GetMergeHelperLaunchSpecAsync(taskIds, listId))));
     }
 
+    // List-handler handoff: the running session called handoff_list_handler at the end of Phase 2.
+    // Opens a SECOND ConPTY tile for the SAME handler task id to carry out Phases 3-5 — deliberately
+    // NOT deduped by TaskId like OpenMergeHelperConPtySessionAsync above, since the original tile is
+    // meant to stay open (Mika closes it by hand once he has seen its last message). No new task is
+    // created here; see InteractiveLaunchSpecService.BuildForMergeHelperHandoffAsync.
+    public async System.Threading.Tasks.Task OpenMergeHelperHandoffConPtySessionAsync(string taskId, IReadOnlyList<string> survivingTaskIds)
+    {
+        if (string.IsNullOrEmpty(taskId) || survivingTaskIds is not { Count: > 0 }) return;
+
+        var baseTitle = Loc.T("missionControl.mergeHelperTitle");
+        var title = baseTitle + Loc.T("missionControl.mergeHelperHandoffTitleSuffix");
+        try
+        {
+            await using var ctx = await _dbFactory.CreateDbContextAsync();
+            var task = await ctx.Tasks.AsNoTracking().FirstOrDefaultAsync(t => t.Id == taskId);
+            if (task is not null)
+            {
+                var list = await ctx.Lists.AsNoTracking().FirstOrDefaultAsync(l => l.Id == task.ListId);
+                if (list?.Name is { Length: > 0 } name)
+                    title = $"{baseTitle} — {name}{Loc.T("missionControl.mergeHelperHandoffTitleSuffix")}";
+            }
+        }
+        catch { /* best-effort title lookup */ }
+
+        AddConPtyPane(new ConPtyPaneViewModel(taskId, title,
+            () => DescribeAsync(() => _worker.GetMergeHelperHandoffLaunchSpecAsync(taskId, survivingTaskIds))));
+    }
+
     // Wires a freshly built pane and shows it immediately — the pane resolves its own launch spec,
     // so the tile is on screen (spinner running) while the worker is still preparing the worktree.
     private void AddConPtyPane(ConPtyPaneViewModel pane)
@@ -327,6 +359,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         _worker.TaskFinishedEvent -= _onTaskFinished;
         _worker.TaskUpdatedEvent -= _onTaskUpdated;
         _worker.ConnectionRestoredEvent -= _onConnectionRestored;
+        _worker.HandoffRequestedEvent -= _onHandoffRequested;
         ConPtySessions.CollectionChanged -= OnConPtySessionsChanged;
         Panes.CollectionChanged -= OnPanesChanged;
         foreach (var c in ConPtySessions.ToList())