fix(macOS): authenticate Git SSH SOCKS proxy connections

[Kaylebor]

Summary

macOS's bundled BSD nc supports SOCKS5 but cannot perform username/password authentication. When the sandbox proxy requires its per-session credential, Git SSH fails during SOCKS method negotiation before reaching the repository.

This adds a small Node-based ProxyCommand client that is selected only on macOS when proxyAuthToken is present. It performs the standard SOCKS5 username/password handshake, connects to the requested SSH destination, and pipes the SSH stream through. Unauthenticated macOS configurations retain the existing BSD nc command, and Linux behavior is unchanged.

The SOCKS server is intentionally untouched. This keeps the change on the client side and avoids the broader server rewrite and hardening scope in #385, which provided the relevant inspiration for the authentication problem.

Testing

• npm run typecheck
• npm run build
• npm run lint:check
• bun test test/sandbox/proxy-env-vars.test.ts test/sandbox/socks-authenticated-client.test.ts
• Live macOS sandbox exercise reached GitHub through the authenticated proxy; the remaining failure was the expected SSH key authorization response rather than proxy authentication.

[Kaylebor]

Reasoning/testing: I've been playing with copying Codex's auto-review flow on Pi (more or less), here https://github.com/Kaylebor/pi-permission-reviewer

Heavily vibe-coded though, not really tested on serious work yet.