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.