stdio client transport: spawns an MCP server as a subprocess and speaks newline-delimited JSON-RPC over its stdin/stdout.

{Noizu.MCP.Client,
 transport: {:stdio, command: "npx", args: ["-y", "@modelcontextprotocol/server-everything"]}}

Options: :command (required), :args, :cd, :env (map or keyword of string pairs).

Erlang ports cannot half-close stdin, so shutdown is Port.close/1 followed by a best-effort SIGTERM to the OS pid. Subprocess stderr is inherited (passes through to the BEAM's stderr).