Add Codex plugin manifest

[yenda]

Summary

• add a Codex-native plugin manifest for Allium
• document the Codex marketplace install commands
• extend the offline skill validation script to catch Codex manifest regressions

Verification

• node scripts/test-skills.mjs
• python3 /home/yenda/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py /home/yenda/allium
• Codex install smoke test through a temporary local marketplace

Notes

This makes the Allium repo itself installable as a Codex plugin. The JUXT marketplace also needs to expose Allium from a Codex-compatible local plugin entry for codex plugin add allium@juxt-plugins to work from juxt/claude-plugins.

[yavorpanayotov]

Verified locally: node scripts/test-skills.mjs codex → 9 passed, 0 failed. The .codex-plugin/plugin.json is well-formed, its version (3.3.0) matches .claude-plugin/plugin.json, and it correctly omits the Claude-only fields (agents/hooks/lspServers) the new codex test group guards against. Good to have that regression check.

Two notes:

• Coupled to Expose Allium as a local marketplace plugin claude-plugins#11. The README's codex plugin marketplace add juxt/claude-plugins / codex plugin add allium@juxt-plugins only works once that marketplace PR lands. Suggest merging the two together so the docs don't get ahead of reality.
• Optional: to keep the two manifests from drifting within this repo, the codex test could assert that name/version/description match .claude-plugin/plugin.json.