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/mcp/tools.py

@@ -0,0 +1,45 @@
+"""Load MCP tools using langchain-mcp-adapters."""
+
+import logging
+
+from langchain_core.tools import BaseTool
+
+from src.config.mcp_config import get_mcp_config
+from src.mcp.client import build_servers_config
+
+logger = logging.getLogger(__name__)
+
+
+async def get_mcp_tools() -> list[BaseTool]:
+    """Get all tools from enabled MCP servers.
+
+    Returns:
+        List of LangChain tools from all enabled MCP servers.
+    """
+    try:
+        from langchain_mcp_adapters.client import MultiServerMCPClient
+    except ImportError:
+        logger.warning("langchain-mcp-adapters not installed. Install it to enable MCP tools: pip install langchain-mcp-adapters")
+        return []
+
+    mcp_config = get_mcp_config()
+    servers_config = build_servers_config(mcp_config)
+
+    if not servers_config:
+        logger.info("No enabled MCP servers configured")
+        return []
+
+    try:
+        # Create the multi-server MCP client
+        logger.info(f"Initializing MCP client with {len(servers_config)} server(s)")
+        client = MultiServerMCPClient(servers_config)
+
+        # Get all tools from all servers
+        tools = await client.get_tools()
+        logger.info(f"Successfully loaded {len(tools)} tool(s) from MCP servers")
+
+        return tools
+
+    except Exception as e:
+        logger.error(f"Failed to load MCP tools: {e}", exc_info=True)
+        return []