Add Cloudflare and Sentry plugins

[vb-openai]

Summary

• add Cloudflare and Sentry plugin folders to openai/plugins
• register both plugins in .agents/plugins/marketplace.json
• vendor the Cloudflare platform, Wrangler, and Agents SDK skills plus the Sentry read-only observability skill
• fix the follow-up review issues in plugin metadata and skill docs

Validation

• parsed .agents/plugins/marketplace.json, plugins/cloudflare/.codex-plugin/plugin.json, and plugins/sentry/.codex-plugin/plugin.json as JSON
• verified the Cloudflare and Sentry defaultPrompt entries are within the documented length cap
• ran git diff --check origin/main...origin/vb/add-missing-plugin-examples

Notes

• GitHub and Notion were removed from this branch because those plugins are already on main
• the Cloudflare vendored reference corpus was whitespace-cleaned as part of the review follow-up