feat: add Codex plugin support without duplicating skills

[federicobartoli]

Summary

Make this repo installable as a Codex plugin with a single command, while keeping skills/ as the single source of truth shared with Claude Code. No files are copied, no installer script, no wrapper skills.

codex plugin marketplace add addyosmani/web-quality-skills

Requires Codex CLI v0.122+ (verified on 0.128.0). On older releases the command was codex marketplace add.

What this adds

• codex/.codex-plugin/plugin.json — Codex manifest ("skills": "./skills/").
• codex/skills — git-tracked symlink to ../skills (9 bytes, mode 120000). Keeps the plugin directory self-contained without duplicating any skill content.
• .agents/plugins/marketplace.json — marketplace entry declaring the plugin at ./codex.
• docs/codex-setup.md — install and usage guide, including a fallback that replaces the symlink with a local copy for Windows users.
• README.md — Codex install section under Quick start.

Net change: 5 files, ~95 lines, 0 duplicated skill content.

Why no duplication

Codex and Claude Code share the same SKILL.md format (YAML frontmatter with name + description). The Codex plugin docs define skills as a relative path to a folder; pointing it at the existing skills/ via a symlink means one file serves both platforms.

Why a codex/ subdirectory

Codex's manifest validator rejects plugins at the marketplace root: source.path must start with ./ and stay inside the marketplace root. codex/ is that subdirectory.

Verified

Tested locally with Codex CLI 0.128.0:

• codex plugin marketplace add /path/to/local/clone succeeds.
• Marketplace registered in ~/.codex/config.toml, cache created under ~/.codex/plugins/cache/web-quality-skills/.
• All 6 skills visible in the plugin's Skills list:
  • web-quality-audit
  • performance
  • core-web-vitals
  • accessibility
  • seo
  • best-practices

Open to alternatives

If a symlink in the repo is undesirable, happy to switch to either:

• Restructure — git mv skills codex/skills (no symlink, but larger diff moving 6 directories), or
• Manual step — drop the symlink and document a cp -R skills codex/skills step after git clone (no symlink, but install is no longer a single codex plugin marketplace add).

Same approach applied to addyosmani/agent-skills#88.

Test plan

• codex plugin marketplace add /path/to/local/clone succeeds on v0.128.0
• Marketplace registered in ~/.codex/config.toml
• Plugin cache created under ~/.codex/plugins/cache/web-quality-skills/
• All 6 skills visible in Codex UI under the plugin's Skills list
• Skills invokable in chat after UI install (pending maintainer verification)

[addyosmani]

LGTM!