Codex attribution

When Codex authors or substantially contributes to a commit in this repository, record that contribution in the commit metadata.

For Codex-authored work, use Codex <codex@openai.com> as the Git author and preserve the human user's configured identity as the committer.
For mixed human and Codex work, preserve the human author and append Co-authored-by: Codex <codex@openai.com> to the commit message.
Do not rewrite existing published history solely to add attribution.