Implement Codex Plugin

Implement Codex plugin packaging: manifest authoring, skill reuse, app integrations, marketplace assets, and distribution to the Codex plugin directory.

Workflow

1. Create .codex-plugin/plugin.json with required name, version, description fields.
2. Wire shared skills/ directory from plugin root (reused across platforms).
3. Optionally add .app.json for third-party app integrations.
4. Add assets/ with marketplace visuals (icon, screenshots).
5. Validate manifest with integration_tests/validate-codex-manifest.sh.
6. Prepare for distribution via Codex plugin directory.

Manifest Schema

{
  "name": "my-plugin",
  "version": "1.0.0",
  "description": "Brief description of what this plugin does",
  "author": { "name": "your-name" },
  "license": "Apache-2.0",
  "repository": "https://github.com/owner/repo",
  "skills": "./skills/"
}