Add MCP server facade module

[knoxHuang]

Summary

• Add new src/lib/mcp/mcp.ts facade module that provides startServer, stopServer, and getStatus APIs for MCP server lifecycle management
• Export the new Mcp module from src/lib/index.ts
• The facade is designed to be called from the utility process, assuming core modules (project/engine/assets/scripting/builder) are already initialized

Test plan

• Verify Mcp.startServer() launches the Express server and registers MCP middleware
• Verify Mcp.stopServer() cleanly shuts down the server
• Verify Mcp.getStatus() returns correct running state and URL
• Verify duplicate startServer() calls return the existing URL without restarting