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。

internal/lua/vm.go

@@ -26,6 +26,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -37,7 +38,7 @@ import (
 	golua "github.com/aarzilli/golua/lua"
 )
 
-//go:embed adapters/*.lua plugins/*.lua
+//go:embed adapters/*.lua
 var bundledAdapters embed.FS
 
 // adapterGlobal is the reserved global holding the adapter table after the
@@ -732,64 +733,24 @@ func (v *VM) Start() error {
 	return nil
 }
 
-// bundledPluginsDir is the directory inside the embedded FS holding the
-// plugins shipped with the gateway. It is separate from adapters/ on purpose:
-// the two are loaded by different machinery into different kinds of Lua state
-// (a protocol transform vs. request-pipeline hooks), and keeping them apart
-// makes it obvious that dropping a file in one does not affect the other.
-const bundledPluginsDir = "plugins"
-
-// ReadBundledPlugin returns the source of a plugin shipped with the gateway.
-// It exists so a test (or an operator tool) can load a bundled plugin without
-// depending on whether seeding has already run for this directory.
+// ReadBundledPlugin is retained only for source-level test fixtures and
+// development tooling. It resolves relative to this source file, not the
+// process working directory, and is never used by production plugin loading.
 func ReadBundledPlugin(name string) (string, error) {
-	data, err := bundledAdapters.ReadFile(bundledPluginsDir + "/" + name + ".lua")
+	_, file, _, ok := runtime.Caller(0)
+	if !ok {
+		return "", fmt.Errorf("locate plugin source %s", name)
+	}
+	data, err := os.ReadFile(filepath.Join(filepath.Dir(file), "plugins", name+".lua"))
 	if err != nil {
-		return "", fmt.Errorf("bundled plugin %s: %w", name, err)
+		return "", fmt.Errorf("plugin source %s: %w", name, err)
 	}
 	return string(data), nil
 }
 
-// writeBundledPlugins seeds the plugin directory with the shipped plugins.
-//
-// It runs only when the directory does not exist yet (same rule as adapters):
-// once the directory exists it is authoritative, so deleting a shipped plugin is
-// a real delete and editing one survives restarts.
-func writeBundledPlugins(dir string) error {
-	if dir == "" {
-		return nil
-	}
-	if err := os.MkdirAll(dir, 0755); err != nil {
-		return fmt.Errorf("mkdir plugin dir: %w", err)
-	}
-	entries, err := bundledAdapters.ReadDir(bundledPluginsDir)
-	if err != nil {
-		return nil // nothing embedded; not an error
-	}
-	for _, e := range entries {
-		if e.IsDir() || filepath.Ext(e.Name()) != ".lua" {
-			continue
-		}
-		dst := filepath.Join(dir, e.Name())
-		// Per-file seeding: a plugin already present is left ALONE. That is what
-		// makes an operator's own copy (or a locally edited one) survive an
-		// upgrade, and it is also why a newly embedded plugin still arrives —
-		// absence of THAT file, not of the directory, is the trigger.
-		if _, err := os.Stat(dst); err == nil {
-			continue
-		} else if !os.IsNotExist(err) {
-			return err
-		}
-		data, err := bundledAdapters.ReadFile(bundledPluginsDir + "/" + e.Name())
-		if err != nil {
-			continue
-		}
-		if err := os.WriteFile(dst, data, 0644); err != nil {
-			return err
-		}
-	}
-	return nil
-}
+// writeBundledPlugins is intentionally a no-op: plugins are independent optional
+// files and are never seeded by the gateway.
+func writeBundledPlugins(string) error { return nil }
 
 func (v *VM) Stop() {
 	select {