Add Codex-native skill package

[daeshawnballard]

Summary

• add a Codex-native Greenlight skill package under codex-skill/
• include SKILL.md + agents/openai.yaml for direct Codex use
• add a short README section with copy/paste setup and invocation

Attribution

• this package adapts the existing Greenlight workflow for Codex
• credit to Lanseer and the Revyl team for the original Greenlight project and workflow

Review checklist

• verify codex-skill/SKILL.md reflects current Greenlight usage
• verify codex-skill/agents/openai.yaml metadata and $app-store-preflight-compliance invocation
• follow README Codex Skill setup steps and confirm skill loads in Codex

Validation

• local skill validation: quick_validate.py codex-skill -> Skill is valid!

[ellipsis-dev]

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