add AGENTS.md to codex

[syphar]

All submissions

• I have followed the Contributing Guidelines
• I have checked to ensure there aren't other open Pull Requests for the same update/change

Adding/updating Application X Support

• This PR is only for one application
• It has been added to the list of supported applications in the README
• The configuration syncs the minimal set of data
• No file specific to the local workstation is synced
• No sensitive data is synced

[syphar]

This is similar to what we already do [for claude-code].

Related documentation:
https://developers.openai.com/codex/guides/agents-md/#create-global-guidance

[lra]

Thanks!