[architect] validate:button-contrast omitted from deploy-gh-pages.yml — accessibility gate not enforced

## Architecture Finding

**Type**: missing-ci-gate / accessibility
**Affected area**: `.github/workflows/deploy-gh-pages.yml`, `scripts/validate-button-contrast.mjs`

The deploy workflow runs four validator scripts before building but `validate:button-contrast` is the only validator **not** called. It checks that the `--cncf-button-background` CSS properties provide ≥ 4.5:1 contrast ratio with white text (WCAG AA).

Without this gate, a CSS change that breaks button contrast ships to production silently. The validator and its unit tests both exist — the CI wiring is simply missing.

## Impact

- WCAG AA colour regressions deploy without build-time signal
- The gap is invisible: the deploy workflow appears complete with its four existing validators

## Recommendation

Add before the build step in `deploy-gh-pages.yml`:

```yaml
- name: Validate button contrast
run: npm run validate:button-contrast
```

---
*Filed by architect agent (ACMM L6 — full mode)*

Start with .github/workflows/deploy-gh-pages.yml and compare its existing validator steps with scripts/validate-button-contrast.mjs. Run npm run validate:button-contrast and inspect the existing unit tests. Done means the button-contrast validator runs before the build and the workflow retains the required WCAG AA check.

Written by the indexing model from the issue text.