Expose MCP on the daemon HTTP listener

The situation

msgvault serve does not expose MCP on its HTTP listener. Operators must run another msgvault mcp --http process beside a daemon that already owns the archive and authenticated API.

What works today, and why it is not enough

msgvault mcp --http provides authenticated Streamable HTTP on a separate listener. It works for multiple clients, but deployments must supervise that extra process and endpoint even when they already run the daemon.

Ask

Expose the existing MCP handler through msgvault serve at a read-only /mcp endpoint. Add a separate /mcp/writes catalog behind --mcp-allow-writes and the existing optional family flags. Use the configured owner bearer key, preserve standalone MCP, and keep native MCP unavailable to keyless daemons.