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>

README.md

@@ -14,6 +14,10 @@ A LangGraph-based AI agent backend with sandbox execution capabilities.
    # Set your API keys
    export OPENAI_API_KEY="your-key-here"
    # or edit config.yaml directly
+
+   # Optional: Enable MCP servers for additional tools
+   cp mcp_config.example.json mcp_config.json
+   # Edit mcp_config.json to enable desired servers
    ```
 
 2. **Install dependencies**:
@@ -45,6 +49,7 @@ deer-flow/
 
 - [Configuration Guide](backend/docs/CONFIGURATION.md) - Setup and configuration instructions
 - [Architecture Overview](backend/CLAUDE.md) - Technical architecture details
+- [MCP Setup Guide](MCP_SETUP.md) - Configure Model Context Protocol servers for additional tools
 
 ## License