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/pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "httpx>=0.28.0",
     "langchain>=1.2.3",
     "langchain-deepseek>=1.0.1",
+    "langchain-mcp-adapters>=0.1.0",
     "langchain-openai>=1.1.7",
     "langgraph>=1.0.6",
     "langgraph-cli[inmem]>=0.4.11",