MCP server

[mathew55]

JSON-RPC interface (stdio transport) so AI agents can query Clash natively via MCP instead of shelling out to the CLI.

Not urgent - agents can already use clash status --json and clash check via Bash. But a nicer interface for the future, especially for tools like clash_check_file, clash_status, and clash_merge_order.

See Section 6 of the technical plan for the full MCP spec.