fix(build): 修好 npm install 挂掉 (postinstall 只重建 better-sqlite3)

`npm install` 在 postinstall 阶段直接失败退出:

    node-hid 编译报错 -> node-addon-api/napi.h:2477 expected unqualified-id

根因不是 node-hid 坏了, 是重建范围错了:

1. postinstall 跑的是 `electron-builder install-app-deps`, 它重建**所有**原生模块;
2. 它给子进程传 `npm_config_build_from_source=false` —— 但这是**字符串** "false",
   npm 11 对未知的 npm_config_* 不做布尔转换;
3. pkg-prebuilds-verify 用 `if (process.env.npm_config_build_from_source)` 判断,
   字符串 "false" 在 JS 里是真值 -> 直接 exit 1 -> 强制源码编译;
4. node-hid 于是去编译它自带的 node-addon-api 3.2.1, 和现代 Node 头文件不兼容 -> 编译失败
   -> 整个 npm install 挂掉。

而 node-hid 根本不需要编译 —— 它自带 N-API 预编译产物, 任何 ABI 都能加载。

真正需要按 Electron ABI 重编的只有 better-sqlite3 (非 N-API, 绑死 V8/ABI,
Electron 32 -> node ABI 128); 其余 (@lydell/node-pty / node-hid /
@serialport/bindings-cpp / usb) 都是 N-API 预编译, 重编既没必要又有害。

改动:

- postinstall 换成 `node scripts/postinstall.js`:
  只重建 better-sqlite3, 装完**在 Electron 下真跑一次读写校验**, 失败就 exit 1
  —— 把"启动即崩"提前到 install 阶段暴露
- package.json 加 `allowScripts` 白名单 (npm 11 新机制, 目前只提醒不拦截,
  但迟早会拦; 顺便让 npm install 输出干净)
- AGENTS.md 坑 #3 改写成新规矩 + 完整根因链
- docs/windows-compat.md / docs/remote-terminal-forward-usb.md 里过时的
  "postinstall 走 install-app-deps" 表述一并纠正

验证:

- npm run build            ✅
- npm run typecheck        ✅ node + web
- npm run selftest         ✅ 64 通过 / 0 失败
- npm run selftest:sync    ✅ 全部通过
- npm run package:linux    ✅ 新 deb
- npm run selftest:package ✅ 4/4 (从 app.asar 真实 spawn pty)

package.json

@@ -6,6 +6,14 @@
   "homepage": "https://github.com/localnetmsg/local-net-msg",
   "author": "LocalNetMsg <dev@localnetmsg.local>",
   "main": "out/main/index.js",
+  "allowScripts": {
+    "better-sqlite3": true,
+    "electron": true,
+    "esbuild": true,
+    "node-hid": true,
+    "@serialport/bindings-cpp": true,
+    "vue-demi": true
+  },
   "scripts": {
     "dev": "electron-vite dev",
     "build": "electron-vite build",
@@ -23,7 +31,7 @@
     "package:mac": "cross-env ELECTRON_MIRROR=https://npmmirror.com/mirrors/electron/ ELECTRON_BUILDER_BINARIES_MIRROR=https://npmmirror.com/mirrors/electron-builder-binaries/ electron-vite build && electron-builder --mac",
     "package:linux": "cross-env ELECTRON_MIRROR=https://npmmirror.com/mirrors/electron/ ELECTRON_BUILDER_BINARIES_MIRROR=https://npmmirror.com/mirrors/electron-builder-binaries/ electron-vite build && electron-builder --linux",
     "package:dir": "cross-env ELECTRON_MIRROR=https://npmmirror.com/mirrors/electron/ ELECTRON_BUILDER_BINARIES_MIRROR=https://npmmirror.com/mirrors/electron-builder-binaries/ electron-vite build && electron-builder --dir",
-    "postinstall": "electron-builder install-app-deps"
+    "postinstall": "node scripts/postinstall.js"
   },
   "dependencies": {
     "@element-plus/icons-vue": "^2.3.1",