Added Codex entrypoints

Property	Value
description	Added thin repo-local Codex instructions that point back to shared MAC governance.
version	0.1.0
type	Create
related	MAC Docusaurus, MAC

• Added a repo-root AGENTS.md entrypoint.
• Added a local .codex/README.md pointer.
• Kept shared governance in mac instead of duplicating it inside MAC Docusaurus.