ci(a11y): use skill-a11y-audit composite action instead of hand-rolling

Drops .pa11yci.json and replaces the pa11y-ci step with a single call to
the new snapsynapse/skill-a11y-audit/.github/actions/scan@main composite
action, which wraps the same axe-based scanner the skill already ships
for non-CI use. The local-server setup and sandbox flag are now the
skill's responsibility, not this repo's.

The "Discover sitemap base URL" step keeps the template flexible so
forks with a different config.url still scan against localhost.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: snapsynapse

.github/workflows/build.yml

@@ -69,28 +69,23 @@ jobs:
       - name: Build site
         run: node scripts/build.js
 
-      - name: Start local server
-        run: |
-          npx --yes http-server ./docs -p 8088 -c-1 > /tmp/http.log 2>&1 &
-          for i in $(seq 1 30); do
-            curl -fs http://localhost:8088/ > /dev/null && break || sleep 0.5
-          done
-          curl -fs http://localhost:8088/ > /dev/null || (cat /tmp/http.log; exit 1)
-
-      - name: Run pa11y-ci (WCAG 2.1 AA)
+      - name: Discover sitemap base URL
+        id: site
         run: |
           if [ -f docs/sitemap.xml ]; then
             SITE_BASE=$(grep -oE '<loc>[^<]+</loc>' docs/sitemap.xml | head -1 | sed -E 's|<loc>(.+)/?</loc>|\1|' | sed -E 's|/$||')
-            npx --yes pa11y-ci \
-              --config .pa11yci.json \
-              --sitemap http://localhost:8088/sitemap.xml \
-              --sitemap-find "$SITE_BASE" \
-              --sitemap-replace 'http://localhost:8088' \
-              --threshold 0
-          else
-            npx --yes pa11y-ci --config .pa11yci.json http://localhost:8088/ --threshold 0
+            echo "base=$SITE_BASE" >> "$GITHUB_OUTPUT"
           fi
 
+      - name: Run skill-a11y-audit (WCAG 2.1 AA)
+        uses: snapsynapse/skill-a11y-audit/.github/actions/scan@main
+        with:
+          serve-path: ./docs
+          sitemap: http://localhost:8088/sitemap.xml
+          sitemap-find: ${{ steps.site.outputs.base }}
+          sitemap-replace: http://localhost:8088
+          fail-on: errors
+
 # GitHub Pages serves directly from `main` branch root on this repo.
 # Template forks that want automated deploy-to-gh-pages can add back the
 # peaceiris/actions-gh-pages step here and set permissions.contents to write.