Add Cloudflare, GitHub, Notion, and Sentry plugins

[vb-openai]

Summary

• add Cloudflare, GitHub, Notion, and Sentry plugin folders to openai/plugins
• normalize their manifests to match current public repo conventions
• register all four plugins in .agents/plugins/marketplace.json in alphabetical order

Validation

• parsed the new plugin manifests and companion .mcp.json / .app.json files as JSON
• verified manifest file references exist for skills, mcpServers, apps, composerIcon, and logo
• verified each new marketplace entry appears exactly once with the expected path and alphabetical placement

Notes

• Cloudflare and Notion bring their full vendored skill packages, including bundled references/examples, because the top-level skill workflows point into those files.