Add measured-design-audit and design-regression-guard (Development & Code Tools)

[Yuxiang-Ma]

Adds two skills to Development & Code Tools, both distilled from real design-engineering sessions (not hypothetical):

• measured-design-audit — design review by measurement instead of eyeballing: computed WCAG contrast against composited backgrounds, 44 px touch targets, font-size/tracking censuses, multi-viewport overflow. In the source sessions it found 133 contrast failures and seventeen distinct font sizes that three visual review rounds had missed; color fixes are solved numerically for the minimum passing shift rather than nudged.
• design-regression-guard — the counterpart: every fixed defect class becomes a Playwright assertion in the same commit (including semantic contracts like "drawn ink equals the reported percentage"). Its credential: the guard caught its own author twice within hours of being written.

Checklist per CONTRIBUTING:

• Real use case: yes — both extracted from multi-day sessions on a live research site.
• No duplicates found: existing entries cover running Playwright (webapp-testing, Playwright Browser Automation) and screenshot capture; neither covers measurement-first auditing nor freezing fixes as regressions.
• Documented with runnable starter blocks in each SKILL.md; tested with Claude Code.
• Entries inserted alphabetically with attribution.