feat(codex): add plugin manifest

[kirillmorozov]

Pull Request

Description

Type of change:

• New content (adding best practices, patterns, or guidance)
• Fix (correcting outdated or incorrect information)
• Refactor (reorganizing or improving clarity)
• Documentation (README, CONTRIBUTING, etc.)
• Testing framework improvement

Summary:

Add Codex plugin manifest to simplify installation process for OpenAI Codex agent.

Testing Evidence (REQUIRED)

I've been able to install this skill as a plugin from my fork using a method I've specified in README.md:

Additional Context

I've also tried to add a marketplace manifest to this repo to be able to install this skill using

codex plugin marketplace add antonbabenko/terraform-skill

but currently it's not possible due to the way this repo is structured and openai/codex#17066.

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For Maintainers

Review Checklist

• Testing evidence is convincing (baseline → compliance improvement shown)
• Standards compliance verified
• Content is accurate and current
• Token efficiency maintained
• Quality standards met
• No conflicts with existing content
• CHANGELOG.md updated (if needed)

Merge Checklist

• Squash commits with clear commit message
• Update CHANGELOG.md if not done in PR
• Consider if version bump needed
• Tag if this completes a planned milestone