New commands:
- $D diff --before old.png --after new.png: visual diff using GPT-4o
vision. Returns differences by area with severity (high/medium/low)
and a matchScore (0-100).
- $D verify --mockup approved.png --screenshot live.png: compares live
site screenshot against approved design mockup. Pass if matchScore
>= 70 and no high-severity differences.
Used by /design-review to close the design loop: design -> implement ->
verify visually.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
