docs(readme): replace stripped <video> with animated WebP demo reel (#441)

GitHub's README sanitizer strips <video> tags sourced from release
assets, so the previous embed never rendered. Swap for an animated
WebP (no audio anyway) at assets/openjarvis_demo_reel.webp — renders
inline on every Markdown viewer with auto-loop.

960px wide, 15fps, lossy q=60, 4.5MB.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -16,7 +16,7 @@
 ---
 
 <div align="center">
-  <video src="https://github.com/open-jarvis/OpenJarvis/releases/download/readme-media/openjarvis_demo_reel_v2.mp4" controls width="100%"></video>
+  <img alt="OpenJarvis demo reel" src="assets/openjarvis_demo_reel.webp" width="100%">
 </div>
 
 ---