fix: harden pi maintenance lifecycle

backend/src/runtime/maintenance-gate.ts

@@ -0,0 +1,10 @@
+import { existsSync } from "node:fs";
+
+/**
+ * A host-side lifecycle transaction creates this marker before it checks for active sessions.
+ * The marker is intentionally only an admission gate: it must never terminate existing Pi
+ * processes or make their in-flight work unavailable.
+ */
+export function createMaintenanceGate(markerFile: string): () => boolean {
+  return () => existsSync(markerFile);
+}