feat(coding-agent): Add MCP extension example

[badlogic]

Summary

Add an extension example that integrates MCP (Model Context Protocol) servers.

Implementation

Configuration

• Read ~/.pi/agent/mcp.json and <cwd>/.pi/mcp.json
• Follow the established format used by Claude Code and other harnesses

MCP Server Connection

• Connect to MCP servers (stdio and SSE transports) using the MCP TypeScript SDK
• Expose each server's tools as custom tools with naming: mcp_<server-name>_<toolname>

/mcp Command

Custom UI component with multiple screens:

1. Server List Screen

   • Show all configured MCP servers
   • Green dot for connected, red X for failed
   • Select a server to view details

2. Server Detail Screen

   • Display server tools
   • Enable/disable server dynamically (adds/removes tools via setActiveTools)