feat: add MCP (Model Context Protocol) support

Add comprehensive MCP integration using langchain-mcp-adapters to enable
pluggable external tools from MCP servers.

Features:
- MCP server configuration via mcp_config.json
- Automatic lazy initialization for seamless use in both FastAPI and LangGraph Studio
- Support for multiple MCP servers (filesystem, postgres, github, brave-search, etc.)
- Environment variable resolution in configuration
- Tool caching mechanism for optimal performance
- Complete documentation and setup guide

Implementation:
- Add src/mcp module with client, tools, and cache components
- Integrate MCP config loading in AppConfig
- Update tool system to include MCP tools automatically
- Add eager initialization in FastAPI lifespan handler
- Add lazy initialization fallback for LangGraph Studio

Dependencies:
- Add langchain-mcp-adapters>=0.1.0

Documentation:
- Add MCP_SETUP.md with comprehensive setup guide
- Update CLAUDE.md with MCP system architecture
- Update config.example.yaml with MCP configuration notes
- Update README.md with MCP setup instructions

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/agents/middlewares/clarification_middleware.py

@@ -41,7 +41,7 @@ class ClarificationMiddleware(AgentMiddleware[ClarificationMiddlewareState]):
         Returns:
             True if text contains Chinese characters
         """
-        return any('\u4e00' <= char <= '\u9fff' for char in text)
+        return any("\u4e00" <= char <= "\u9fff" for char in text)
 
     def _format_clarification_message(self, args: dict) -> str:
         """Format the clarification arguments into a user-friendly message.