New `$D extract` command: sends approved mockup to GPT-4o vision,
extracts color palette, typography, spacing, and layout patterns,
writes/updates DESIGN.md with an "Extracted Design Language" section.
Progressive constraint: if DESIGN.md exists, future mockup briefs
include it as style context. If no DESIGN.md, explorations run wide.
readDesignConstraints() reads existing DESIGN.md for brief construction.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Garry Tan