feat: move proxmox discovery/import/sync into a first-party plugin

plugins/proxmox/backend/proxmox-jump-hosts.ts

@@ -0,0 +1,17 @@
+export function parseProxmoxJumpHosts(raw: unknown): unknown[] | null {
+  if (!raw) return null;
+  if (Array.isArray(raw)) return raw;
+  if (typeof raw !== "string") return null;
+
+  try {
+    const parsed = JSON.parse(raw);
+    return Array.isArray(parsed) ? parsed : null;
+  } catch {
+    return null;
+  }
+}
+
+export function serializeProxmoxJumpHosts(raw: unknown): string | null {
+  const parsed = parseProxmoxJumpHosts(raw);
+  return parsed ? JSON.stringify(parsed) : null;
+}