fix(ui): failureReason "usage_limit" nachziehen + Worktree-Gate prüft die Platte

Zwei liegengebliebene Consumer aus den letzten beiden Commits:

- TaskRunner klassifiziert seit 07dd7570 "usage_limit", aber weder
  TaskRowViewModel.FailureReasonLabel noch vm.failureReason (de/en) noch
  die get_task-Tool-Beschreibung kannten den Wert — die UI zeigte
  "Grund unbekannt", das MCP-Doc listete weiterhin max_turns|timeout|error.
- TaskRowViewModel.CanOpenWorktree prüfte nur auf einen nicht-leeren
  String. Die Zeile behält den Path eines gemergten/verworfenen Worktrees,
  also war der Menüpunkt aktiv und Process.Start warf in den Footer.
  Jetzt zusätzlich Directory.Exists — dieselbe Prüfung, die
  WorktreesOverviewModalViewModel und MergeSectionViewModel schon machen.

src/ClaudeDo.Localization/locales/en.json

@@ -719,7 +719,7 @@
     "mainWindow": { "closeConfirm": { "message": "{0} Mission Control session(s) are still running. Closing the app will end them. Close anyway?" } },
     "agentStatus": { "idle": "Idle", "queued": "Queued", "running": "Running", "review": "Review", "children": "Waiting for Subtasks", "done": "Done", "failed": "Failed", "cancelled": "Cancelled" },
     "taskStatus": { "idle": "Idle", "queued": "Queued", "running": "Running", "waitingForReview": "Waiting for Review", "waitingForChildren": "Waiting for Subtasks", "done": "Done", "failed": "Failed", "cancelled": "Cancelled", "parked": "Parked", "interactive": "Interactive" },
-    "failureReason": { "maxTurns": "Turn limit reached", "timeout": "Timed out", "error": "Error", "cancelled": "Cancelled", "unknown": "Unknown reason" },
+    "failureReason": { "maxTurns": "Turn limit reached", "timeout": "Timed out", "usageLimit": "Usage limit reached", "error": "Error", "cancelled": "Cancelled", "unknown": "Unknown reason" },
     "failureReasonTooltip": { "maxTurns": "Turn limit reached ({0}/{1} turns) — the worktree is usually fine; continue the task instead of resetting it." },
     "planningBadge": { "active": "PLANNING", "finalized": "PLANNED" },
     "taskRow": { "createdPrefix": "Created {0}", "stepsText": "{0}/{1} steps" },