docs: add Trendshift badge to README header

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012SSMrNKRooMGmJ1yNFpLMy

Author: JuliusBrussee

README.md

@@ -12,6 +12,10 @@
   <strong>8.5%</strong> on <a href="#independently-measured-jetbrains-86-tasks">long-horizon agentic coding runs</a>. Brain still big. Mouth small.
 </p>
 
+<p align="center">
+  <a href="https://trendshift.io/repositories/25391?utm_source=repository-badge&amp;utm_medium=badge&amp;utm_campaign=badge-repository-25391" target="_blank" rel="noopener noreferrer"><img src="https://trendshift.io/api/badge/repositories/25391" alt="JuliusBrussee%2Fcaveman | Trendshift" width="250" height="55"/></a>
+</p>
+
 <p align="center">
   <a href="https://github.com/JuliusBrussee/caveman/stargazers"><img src="https://img.shields.io/github/stars/JuliusBrussee/caveman?style=flat&color=yellow" alt="Stars"></a>
   <a href="./INSTALL.md"><img src="https://img.shields.io/badge/works_with-30%2B_agents-orange?style=flat" alt="30+ agents"></a>