Refactor MCPAgent to maintain a persistent external server process.\n\nInstead of starting and stopping the MCP server for each request, the agent\nnow uses sync.Once for lazy initialization on the first call.\nIt stores the running exec.Cmd, stdio pipes, and the mcp.Client\nin the agent struct.\n\nA sync.Mutex protects concurrent access to the client/pipes.\nA goroutine monitors the process using cmd.Wait() and logs if it exits\nunexpectedly.\n\nThis avoids the overhead of process creation/destruction on every\nmetadata retrieval request.
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.
