Website design system

Owns the how it looks and performs visually layer: mobile graphics, dark mode, and the theme tokens. Outputs/maintains BRAND.md (template: the new-website skill's templates/brand.md) and the token block in that skill's templates/astro/src/styles/global.css. Image rules are below (WebP/AVIF via Astro's astro:assets); the images.spec.ts test enforces them. For generating image assets, use the image skill.

Dark mode (first-class)