feat: add /a11y-check skill — WCAG 2.1 AA accessibility audit for web apps

### /a11y-check — Accessibility audit skill

I use gstack on a few frontend projects and `/seo-audit` is great for discoverability, but I keep having to run a11y checks manually in separate tools (axe-core CLI, Lighthouse).

Would be useful to have a `/a11y-check` skill that audits a web app for WCAG 2.1 AA compliance — things like:
- Missing alt text on images
- Color contrast ratios below 4.5:1
- Interactive elements without keyboard focus
- Missing ARIA labels on form inputs
- Heading hierarchy issues

**Use case:** After any UI change during `/ship`, run `/a11y-check` on the preview URL before merging. Catches regressions that are easy to introduce but annoying to find later.

**Possible approach:** Could shell out to axe-core CLI (`npx axe-cli `) or parse Lighthouse JSON output. The `/browse` skill already handles headless browsing, so maybe it can piggyback on that infrastructure.

Happy to contribute if you think this direction makes sense.

Start by reviewing the existing /browse and /seo-audit skills, especially how /browse handles headless browsing. Define the /a11y-check scope around the listed WCAG 2.1 AA checks and its use during /ship, then validate the audit against a preview URL using either axe-core CLI or Lighthouse JSON output.

Written by the indexing model from the issue text.