Reverse-engineer a website's visual design system from a URL and produce a fully spec-compliant DESIGN.md file (https://github.com/google-labs-code/design.md). The output includes both machine-readable YAML design tokens (colors, typography, spacing, rounded corners, components) and human-readable markdown rationale sections (Overview, Colors, Typography, Layout, Elevation & Depth, Shapes, Components, Do's and Don'ts). Use this skill whenever the user wants to generate a DESIGN.md, create a design system file from a website, capture a site's visual identity, extract design tokens, build a design spec from a URL, clone a site's look and feel, or scaffold a DESIGN.md from scratch. Also triggers on: 'design system from URL', 'generate DESIGN.md', 'extract colors from site', 'what are this site's design tokens', 'capture design from website'.