Add a new agent implementation MCPAgent in core/agents/mcp.\n\nThis agent interacts with an external Model Context Protocol (MCP) server\nusing the github.com/metoro-io/mcp-golang library via stdio.\n\nIt currently implements the ArtistBiographyRetriever interface to fetch\nbiographies by calling the 'get_artist_biography' tool on the MCP server.\nThe path to the MCP server executable is hardcoded for this PoC.\n\nIncludes basic Ginkgo test setup for the new agent.