Added repo-root AGENTS.md to onboard OpenAI Codex as a coding agent. The file mirrors CLAUDE.md, instructs Codex to read and follow .github/copilot-instructions.md, enforces sequential doc loading, the [LOADED_DOCS: ...] prefix, and maps Copilot tool names to Codex shell actions. No protocol rules changed; enables Codex compliance across all primary repos.
