feat: add benchmark demo GIF to README hero section

Replace static VLM/chat screenshots with auto-playing benchmark demo GIF.
Shows HomeSec-Bench running live with Qwen3.5 on Apple M5 Pro.
GIF is 8.3 MB (under GitHub 10 MB limit), auto-plays on GitHub.

Author: solderzzc

README.md

@@ -36,7 +36,9 @@
 
 </div>
 
-<video src="screenshots/aegis-benchmark-demo.mp4" autoplay loop muted playsinline width="100%"></video>
+<p align="center">
+<img src="screenshots/aegis-benchmark-demo.gif" alt="Aegis AI Benchmark Demo — Local LLM home security on Apple Silicon" width="100%">
+</p>
 
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