Add Codex plugin support, update URLs to Shopify org, restore Copilot section

- Add plugins/shopify-plugin/.codex-plugin/plugin.json
- Add .agents/plugins/marketplace.json for Codex marketplace
- Update all repo URLs from shopify-playground to Shopify org
- Fix Claude Code install command to shopify-plugin@shopify-ai-toolkit
- Restore GitHub Copilot/VS Code installation section in README
- Add Codex installation section to README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>