docs(readme): add Superagent badge and move Star History rank badge to its own line (#15460)

- Add the Superagent security badge to the README badge row, with light and dark variants via <picture>.
- Move the Star History global rank badge into its own centered paragraph below the badge row, with spacing.

Co-Authored-By: Amy (Opus) via Paperclip <claude@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -14,8 +14,12 @@
 <p align="center">
   <a href="https://github.com/paperclipai/paperclip/blob/master/LICENSE"><img src="https://img.shields.io/badge/license-MIT-blue" alt="MIT License" /></a>
   <a href="https://github.com/paperclipai/paperclip/stargazers"><img src="https://img.shields.io/github/stars/paperclipai/paperclip?style=flat" alt="Stars" /></a>
-  <a href="https://www.star-history.com/paperclipai/paperclip"><img src="https://api.star-history.com/badge?repo=paperclipai/paperclip" alt="Star History Rank" /></a>
   <a href="https://discord.gg/m4HZY7xNG3"><img src="https://img.shields.io/badge/discord-join-7289da" alt="Discord" /></a>
+  <a href="https://superagent.sh"><picture><source media="(prefers-color-scheme: dark)" srcset="https://www.superagent.sh/api/badge/paperclipai/paperclip/shield.svg?theme=dark&accent=teal&variant=scanned" /><img src="https://www.superagent.sh/api/badge/paperclipai/paperclip/shield.svg?theme=light&accent=teal&variant=scanned" alt="Security posture verified by Superagent" /></picture></a>
+</p>
+
+<p align="center">
+  <a href="https://www.star-history.com/paperclipai/paperclip"><img src="https://api.star-history.com/badge?repo=paperclipai/paperclip" alt="Star History Rank" /></a>
 </p>
 
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