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/config/app_config.py

@@ -6,6 +6,7 @@ import yaml
 from dotenv import load_dotenv
 from pydantic import BaseModel, ConfigDict, Field
 
+from src.config.mcp_config import McpConfig
 from src.config.model_config import ModelConfig
 from src.config.sandbox_config import SandboxConfig
 from src.config.skills_config import SkillsConfig
@@ -24,6 +25,7 @@ class AppConfig(BaseModel):
     tools: list[ToolConfig] = Field(default_factory=list, description="Available tools")
     tool_groups: list[ToolGroupConfig] = Field(default_factory=list, description="Available tool groups")
     skills: SkillsConfig = Field(default_factory=SkillsConfig, description="Skills configuration")
+    mcp: McpConfig = Field(default_factory=McpConfig, description="MCP configuration")
     model_config = ConfigDict(extra="allow", frozen=False)
 
     @classmethod
@@ -80,6 +82,10 @@ class AppConfig(BaseModel):
         if "summarization" in config_data:
             load_summarization_config_from_dict(config_data["summarization"])
 
+        # Load MCP config separately (it's in a different file)
+        mcp_config = McpConfig.from_file()
+        config_data["mcp"] = mcp_config.model_dump()
+
         result = cls.model_validate(config_data)
         return result