## What changed
Expose the shared `Child` type and `spawn_child` function from `codex-utils-pty`, and update the MCP stdio transport to use them. Move the existing macOS spawning implementation and its tests into the utility crate, preserving platform-specific spawning, piped stdio, and kill-on-drop behavior.
GitOrigin-RevId: 26c2fb2f22cdba907aa9d18cab7dc8c4ac1c909e