fix: CI 改用 pnpm 解决 npm 'Exit handler never called' 崩溃

GitHub runner 上 npm install/ci 反复 'Exit handler never called!' 崩溃
（72s 无输出后退出），改用 pnpm 9 + Node 22：
- pnpm/action-setup@v4 + setup-node cache pnpm
- 生成 pnpm-lock.yaml，删除 package-lock.json
- package.json 加 packageManager 字段，scripts 统一 pnpm

site/package.json

@@ -4,11 +4,12 @@
   "type": "module",
   "scripts": {
     "gen": "node scripts/build.mjs",
-    "dev": "npm run gen && vitepress dev",
-    "build": "npm run gen && vitepress build",
+    "dev": "pnpm run gen && vitepress dev",
+    "build": "pnpm run gen && vitepress build",
     "preview": "vitepress preview"
   },
   "devDependencies": {
     "vitepress": "^1.5.0"
-  }
-}
+  },
+  "packageManager": "pnpm@9.15.9"
+}