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/client.py

@@ -0,0 +1,57 @@
+"""MCP client using langchain-mcp-adapters."""
+
+import logging
+from typing import Any
+
+from src.config.mcp_config import McpConfig, McpServerConfig
+
+logger = logging.getLogger(__name__)
+
+
+def build_server_params(server_name: str, config: McpServerConfig) -> dict[str, Any]:
+    """Build server parameters for MultiServerMCPClient.
+
+    Args:
+        server_name: Name of the MCP server.
+        config: Configuration for the MCP server.
+
+    Returns:
+        Dictionary of server parameters for langchain-mcp-adapters.
+    """
+    params: dict[str, Any] = {
+        "command": config.command,
+        "args": config.args,
+        "transport": "stdio",  # Default to stdio transport
+    }
+
+    # Add environment variables if present
+    if config.env:
+        params["env"] = config.env
+
+    return params
+
+
+def build_servers_config(mcp_config: McpConfig) -> dict[str, dict[str, Any]]:
+    """Build servers configuration for MultiServerMCPClient.
+
+    Args:
+        mcp_config: MCP configuration containing all servers.
+
+    Returns:
+        Dictionary mapping server names to their parameters.
+    """
+    enabled_servers = mcp_config.get_enabled_servers()
+
+    if not enabled_servers:
+        logger.info("No enabled MCP servers found")
+        return {}
+
+    servers_config = {}
+    for server_name, server_config in enabled_servers.items():
+        try:
+            servers_config[server_name] = build_server_params(server_name, server_config)
+            logger.info(f"Configured MCP server: {server_name}")
+        except Exception as e:
+            logger.error(f"Failed to configure MCP server '{server_name}': {e}")
+
+    return servers_config