feat(worker): Handler-Session nach Usage-Limit-Reset automatisch neu starten

Eine List-Handler-ConPTY-Session, die mitten in einer Phase stirbt, war bisher
ein stilles totes Tile: InteractiveTerminalViewModel.OnSessionProcessExited
setzte nur bei einem Tod innerhalb des 5s-Grace-Fensters ein sichtbares Banner.

Neu:
- HandlerRoundState (JSON unter ~/.claudeDo/handler-rounds.json, keine
  EF-Migration): Phase + Task-Set der laufenden Runde, geschrieben von
  BuildForMergeHelperAsync ("triage") und BuildForMergeHelperHandoffAsync.
  Ohne das existieren beide nur im transienten HandoffRequested-Broadcast.
- HandlerRestartCoordinator: entscheidet den Neustart, fail-closed. Beleg fuer
  "war das Usage-Limit" ist FiveHour.Utilization >= 95 zum Todeszeitpunkt --
  eine ConPTY-Session hat kein stdout, TaskRunner.IsUsageLimitMessage hat hier
  nichts zu lesen. Nur bei PowerUse (beide Keys), max ein Neustart pro Runde,
  Warten bis FiveHour.ResetsAt, dann HandoffRequested erneut broadcasten.
  Verbraucht kein Handoff-Budget (HandoffRoundTracker.Register wird nicht
  angefasst). WorkerLog-Zeilen beim Tod und beim Neustart.
- Ui meldet den Exit-Code (nur dort bekannt) via
  WorkerHub.ReportInteractiveSessionExited, ohne selbst zu filtern.

Sibling zu UsageLimitAutoContinueCoordinator, kein Umbau davon: der sieht nur
headless Runs.

src/ClaudeDo.Ui/ViewModels/MissionControlViewModel.cs

@@ -413,10 +413,28 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         pane.ErrorReported += OnConPtyPaneError;
         pane.CloseRequested += CloseConPtySession;
         pane.SubmitForReviewRequested += OnPaneSubmitForReview;
+        pane.SessionDiedAfterStartup += OnPaneSessionDied;
         ConPtySessions.Add(pane);
         pane.Start();
     }
 
+    // A ConPTY session that had been running died. The UI process owns the terminal, so this is the
+    // only place the exit code exists -- report it and let the worker decide whether it was a list
+    // handler killed by the usage limit and worth reopening after the reset
+    // (HandlerRestartCoordinator). Every other pane is just a no-op there, so no filtering here.
+    private async void OnPaneSessionDied(string taskId, int exitCode)
+    {
+        try
+        {
+            await _worker.ReportInteractiveSessionExitedAsync(taskId, exitCode);
+        }
+        catch (Exception ex)
+        {
+            // A worker that is down cannot restart anything anyway; the dead tile is already visible.
+            ErrorReported?.Invoke(Loc.T("missionControl.conptyLaunchFailed", ex.Message));
+        }
+    }
+
     private static async System.Threading.Tasks.Task<TerminalLaunchDescriptor> DescribeAsync(
         Func<System.Threading.Tasks.Task<LaunchSpec>> fetch)
     {
@@ -457,6 +475,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
         pane.ErrorReported -= OnConPtyPaneError;
         pane.CloseRequested -= CloseConPtySession;
         pane.SubmitForReviewRequested -= OnPaneSubmitForReview;
+        pane.SessionDiedAfterStartup -= OnPaneSessionDied;
         ConPtySessions.Remove(pane);
         pane.Dispose();
     }
@@ -507,6 +526,7 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
             c.ErrorReported -= OnConPtyPaneError;
             c.CloseRequested -= CloseConPtySession;
             c.SubmitForReviewRequested -= OnPaneSubmitForReview;
+            c.SessionDiedAfterStartup -= OnPaneSessionDied;
             c.Dispose();
         }
         ConPtySessions.Clear();