Create AGENTS.md for Codex repo instructions

[ethanolivertroy]

Context

The repository currently has CLAUDE.md, which provides Claude Code-specific guidance. Codex conventionally uses repository instructions such as AGENTS.md for agent behavior, commands, testing, and project-specific constraints.

Tasks

• Create root AGENTS.md for Codex.
• Port relevant content from CLAUDE.md while removing Claude Code-specific assumptions.
• Document supported shell commands and test commands from package.json.
• Explain the plugin directory structure in Codex-neutral terms.
• Add guardrails for GRC/security/compliance output:
  • do not reproduce copyrighted framework text beyond allowed excerpts
  • cite SCF as crosswalk/reference where applicable
  • preserve evidence provenance and chain-of-custody
• Include migration warning: .claude-plugin content is source material until Codex compatibility is implemented.

Acceptance Criteria

• Codex can use AGENTS.md as its primary repo instruction file.
• CLAUDE.md may remain for source compatibility, but Codex users are directed to AGENTS.md.