Add ssh-agent-socket subcommand

[rcdailey]

Summary

• Add rbw ssh-agent-socket subcommand that prints the SSH agent socket path
• Makes SSH_AUTH_SOCK setup platform-agnostic, especially for macOS where
  XDG_RUNTIME_DIR is unset and the fallback path through TMPDIR is opaque

Usage becomes:

export SSH_AUTH_SOCK="$(rbw ssh-agent-socket)"

Relates to #295.

[rcdailey]

The lint failure is a pre-existing cargo deny advisory (RUSTSEC-2026-0007, integer overflow in BytesMut::reserve) unrelated to this PR. The advisory was published after the last main branch CI run on Jan 10, so main would hit the same failure today.