Support Codex Agent Plugin Marketplace Fotmat

https://developers.openai.com/plugins/build/plugins#marketplace-metadata

The Codex's Agent Plugin Marketplace metadata is defined in `$REPO_ROOT/.agents/plugins/marketplace.json`, which is an agentagnostic way because no mentioning of OpenAI trade marks (unlike Claude's `.claude-plugin/marketplace.json` or Copilot's `.github/plugin/marketplace.json`)

While Copilot already supports Claude's, it would be even nicer if the Codex's implementation of vendor agnostic marketplace definition can be supported.

Ironically, the only company (Anthropic) who doesn't participate in the [Agent Plugins project](https://agent-plugins.org/) got their proprietary marketplace metadata format universally supported by all participants' coding agent.