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/MissionControl/ConPtyPaneViewModel.cs

@@ -36,6 +36,12 @@ public sealed partial class ConPtyPaneViewModel : ViewModelBase, IDisposable
     /// commits the worktree and moves the task to WaitingForReview.</summary>
     public event Action<string>? SubmitForReviewRequested;
 
+    /// <summary>Raised with <c>(taskId, exitCode)</c> when a task-based session that had been running
+    /// dies non-zero. The host reports it to the worker, which decides whether this was a list
+    /// handler killed by the usage limit and worth reopening. Ad-hoc panes never raise it — there is
+    /// no task to reopen.</summary>
+    public event Action<string, int>? SessionDiedAfterStartup;
+
     /// <summary>Task-based pane — dedup'd by <see cref="TaskId"/>. Pass null for an ad-hoc pane
     /// (no task, never deduped); prefer <see cref="CreateAdHoc"/> at ad-hoc call sites.
     /// <paramref name="descriptorFactory"/> is resolved by the pane itself (in <see cref="Start"/>)
@@ -49,7 +55,34 @@ public sealed partial class ConPtyPaneViewModel : ViewModelBase, IDisposable
         TaskId = taskId;
         _displayTitle = displayTitle;
         _descriptorFactory = descriptorFactory;
-        Terminal.PropertyChanged += OnTerminalPropertyChanged;
+        Subscribe(Terminal);
+    }
+
+    // Terminal is swapped wholesale on Retry, so the handlers follow the property rather than being
+    // wired once at construction. (The initial value comes from the field initializer, which never
+    // runs through the setter -- hence the explicit Subscribe in the constructor.)
+    partial void OnTerminalChanging(InteractiveTerminalViewModel? oldValue, InteractiveTerminalViewModel newValue)
+    {
+        if (oldValue is not null) Unsubscribe(oldValue);
+    }
+
+    partial void OnTerminalChanged(InteractiveTerminalViewModel value) => Subscribe(value);
+
+    private void Subscribe(InteractiveTerminalViewModel terminal)
+    {
+        terminal.PropertyChanged += OnTerminalPropertyChanged;
+        terminal.ExitedAfterStartup += OnTerminalExitedAfterStartup;
+    }
+
+    private void Unsubscribe(InteractiveTerminalViewModel terminal)
+    {
+        terminal.PropertyChanged -= OnTerminalPropertyChanged;
+        terminal.ExitedAfterStartup -= OnTerminalExitedAfterStartup;
+    }
+
+    private void OnTerminalExitedAfterStartup(int exitCode)
+    {
+        if (TaskId is { } id) SessionDiedAfterStartup?.Invoke(id, exitCode);
     }
 
     private readonly Func<System.Threading.Tasks.Task<TerminalLaunchDescriptor>> _descriptorFactory;
@@ -111,16 +144,15 @@ public sealed partial class ConPtyPaneViewModel : ViewModelBase, IDisposable
     [RelayCommand(CanExecute = nameof(CanRetry))]
     private void Retry()
     {
-        Terminal.PropertyChanged -= OnTerminalPropertyChanged;
-        Terminal.Dispose();
-        Terminal = new InteractiveTerminalViewModel();
-        Terminal.PropertyChanged += OnTerminalPropertyChanged;
+        var dead = Terminal;
+        Terminal = new InteractiveTerminalViewModel(); // OnTerminalChanging/Changed move the handlers
+        dead.Dispose();
         Start();
     }
 
     public void Dispose()
     {
-        Terminal.PropertyChanged -= OnTerminalPropertyChanged;
+        Unsubscribe(Terminal);
         Terminal.Kill();
         Terminal.Dispose();
     }