Adds a Codex plugin (.codex-plugin/plugin.json) and repo marketplace
(.agents/plugins/marketplace.json) alongside the existing Claude Code
plugin, so users can install with:
codex plugin marketplace add Aaronontheweb/dotnet-skills
codex plugin add dotnet-skills@dotnet-skills
Skills are auto-discovered from ./skills/ - no skill changes needed.
Adds scripts/validate-codex-plugin.sh to enforce frontmatter, version
lockstep with the Claude plugin, and marketplace path resolution, wired
into CI. Closes#75.