Add a demo GIF to the top of the README

[macOS26]

What

The README leads with a static screenshot. A short (10–20s) animated GIF showing Agent! doing something Claude Code/Cursor can't — e.g. building an Xcode project, driving Safari, or automating a Mac app via Accessibility — would dramatically improve first impressions.

Recipe

1. Build Agent! from source (./build.sh — see CONTRIBUTING.md, no Apple Developer account needed) or download a release
2. Record a short screen capture of a compelling task (QuickTime or screencapture)
3. Convert to an optimized GIF (e.g. ffmpeg + gifsicle), keep it under ~10 MB
4. Drag-and-drop the GIF into a PR editing README.md, placing it right below the badges

Definition of done

• GIF appears near the top of the README, under 10 MB, shows a real task end-to-end

No Swift knowledge required — this is a docs/media contribution.