feat(lua)!: 插件只通过 modelrouter SDK 与内核交互，且不再随内核安装

- 新增唯一公开接口表 `modelrouter`（sdk_version=1.0）：http / secrets /
  kernel / adapters / plugin_dir。成员与内部桥是**同一对象**而非副本 ——
  复制会让凭据保险库分叉（插件写 A、内核读 B）。
- `llmsproxy` 与 `__llmsproxy_*` 降为同对象兼容别名，只为已装插件继续可用。
  仓库内 4 个插件（channels / billing / quota / usage）全部改用
  `modelrouter.*`：此前它们直接依赖内核私有全局，内核无法在不炸插件的前提下
  改实现（重命名、加守卫、换实现都算破坏性变更）。
- 插件不再由网关安装：`SeedBundled()` 变彻底 no-op，`core.New` 不再调用；
  `ReadBundledPlugin` 改为按源文件定位（脱离 cwd），只服务测试与开发工具。
- 桌面端删除 `BUILTIN_PLUGINS` 与 `syncBundledPluginsOnUpgrade`：升级不再
  删除、改名或覆盖用户插件文件。插件是运维资产，不是内核资源。
- `Builtin` 不再靠"源码与仓库示例逐字节相同"推断，插件一律视为外部文件。
- 判据：SDK 存在/版本/形状、别名与内部桥同一张表、适配器 VM 拿不到 SDK、
  经 SDK 真实写读凭据并驱动内核动作、GUI 升级不动插件文件、release 包不含插件。

说明：channels.lua / quota.lua 同时带有此前的渠道与额度改动（含 OpenCode 免费池
监控），与 SDK 改名同处一个文件，无法文件级拆分，故一并纳入。

cmd/gui/plugin-sync.js

@@ -1,18 +1,11 @@
-// Plugin-tree maintenance for the embedded core (no Electron imports, so this
+// Plugin-tree maintenance for the desktop profile (no Electron imports, so this
 // is unit-testable with plain node).
 //
-// Two problems this solves, both invisible from the gateway side:
-//   1. The core deliberately does NOT default `plugin_dir` (an operator opts
-//      in). A desktop profile without it runs with ZERO plugins — the channels
-//      plugin (account login, credential vault, auto check-in) never appears.
-//   2. The core seeds bundled plugins only when the file is ABSENT, so a
-//      desktop upgrade would keep the user's old channels.lua forever and
-//      silently skip every channel fix. On version change the shell must drop
-//      the built-in files so the new binary reseeds them.
+// The desktop may configure an operator-selected plugin directory, but it never
+// owns the files in that directory. In particular, upgrades must not seed,
+// replace, rename, or delete plugins. Plugin installation is an explicit
+// operator action (or an external package manager action).
 const fs = require("fs");
-const path = require("path");
-
-const BUILTIN_PLUGINS = ["billing.lua", "channels.lua", "quota.lua", "usage.lua"];
 
 // ensurePluginDirConfigured back-fills `plugin_dir` into an existing config.
 // Appending a top-level YAML key is safe for the shapes this file can have
@@ -25,7 +18,7 @@ function ensurePluginDirConfigured(configFile, pluginsDir, log) {
     fs.writeFileSync(
       configFile,
       cur.replace(/\s*$/, "") +
-        "\n# added by ModelRouter Desktop: enables the plugin tree (channels etc.)\n" +
+        "\n# added by ModelRouter Desktop: enables the operator plugin directory\n" +
         "plugin_dir: " + JSON.stringify(pluginsDir) + "\n",
       "utf-8",
     );
@@ -37,46 +30,4 @@ function ensurePluginDirConfigured(configFile, pluginsDir, log) {
   }
 }
 
-// syncBundledPluginsOnUpgrade drops built-in plugin files when the app version
-// changed, so the core reseeds them from the binary it shipped with. Returns
-// { changed, dropped:[...], kept: "prev-<ver>" }.
-function syncBundledPluginsOnUpgrade(profileDir, pluginsDir, version, log) {
-  const stamp = path.join(profileDir, ".plugin-seed-version");
-  let have = "";
-  try {
-    have = fs.readFileSync(stamp, "utf-8").trim();
-  } catch (e) {
-    /* first run */
-  }
-  if (have === version) return { changed: false, dropped: [] };
-  const dropped = [];
-  for (const name of BUILTIN_PLUGINS) {
-    const f = path.join(pluginsDir, name);
-    if (!fs.existsSync(f)) continue;
-    try {
-      if (have) fs.renameSync(f, f + ".prev-" + have);
-      else fs.unlinkSync(f);
-      dropped.push(name);
-    } catch (e) {
-      if (log) log("[gui] drop builtin plugin " + name + " failed: " + e.message);
-    }
-  }
-  try {
-    fs.writeFileSync(stamp, version + "\n", "utf-8");
-  } catch (e) {
-    if (log) log("[gui] write plugin seed stamp failed: " + e.message);
-  }
-  if (dropped.length && log) {
-    log(
-      "[gui] plugins refreshed for " + version + " (" + dropped.length +
-        " builtin file(s) reseeded by core)",
-    );
-  }
-  return { changed: true, dropped, kept: have ? "prev-" + have : "" };
-}
-
-module.exports = {
-  BUILTIN_PLUGINS,
-  ensurePluginDirConfigured,
-  syncBundledPluginsOnUpgrade,
-};
+module.exports = { ensurePluginDirConfigured };