feat: 添加 GrowthBook 自定义服务器适配器

通过 CLAUDE_GB_ADAPTER_URL/KEY 环境变量连接自定义 GrowthBook 实例，
无配置时所有 feature 读取返回代码默认值。支持 GrowthBook Cloud（非 remoteEval），
含完整文档和 feature key 列表。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/constants/keys.ts

@@ -3,6 +3,10 @@ import { isEnvTruthy } from '../utils/envUtils.js'
 // Lazy read so ENABLE_GROWTHBOOK_DEV from globalSettings.env (applied after
 // module load) is picked up. USER_TYPE is a build-time define so it's safe.
 export function getGrowthBookClientKey(): string {
+  // 适配器优先：自定义 GrowthBook 服务器
+  const adapterKey = process.env.CLAUDE_GB_ADAPTER_KEY
+  if (adapterKey) return adapterKey
+
   return process.env.USER_TYPE === 'ant'
     ? isEnvTruthy(process.env.ENABLE_GROWTHBOOK_DEV)
       ? 'sdk-yZQvlplybuXjYh6L'