refactor: improve code quality and fix linting issues

- Derive Default trait instead of manual implementation for McpRoot and ProviderManager
- Remove redundant closures in codex_config.rs and config.rs
- Simplify match statements to if let patterns in migration.rs and lib.rs
- Remove unnecessary type conversions and borrows in lib.rs
- Fix i18n key inconsistency: sequentialThinking → sequential-thinking
- Format TypeScript files to match Prettier style

All clippy warnings resolved, code passes all quality checks.

src/i18n/locales/zh.json

@@ -375,7 +375,7 @@
         "name": "@modelcontextprotocol/server-memory",
         "description": "知识图谱记忆系统，支持存储实体、关系和观察，让 AI 记住对话中的重要信息"
       },
-      "sequentialThinking": {
+      "sequential-thinking": {
         "name": "@modelcontextprotocol/server-sequential-thinking",
         "description": "顺序思考工具，帮助 AI 将复杂问题分解为多个步骤，逐步深入思考"
       },