Fix badge cache purge by using GitHub API instead of scraping page HTML

GitHub's repository page is JS-rendered, so curl gets a skeleton without
README images. Use the rendered README API endpoint instead, which returns
actual HTML including camo-proxied image URLs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -18,7 +18,9 @@ jobs:
         run: shellcheck systab
       - name: Purge GitHub mirror badge cache
         run: |
-          curl -sL https://github.com/opennomad/systab \
+          curl -sL \
+            -H "Accept: application/vnd.github.html+json" \
+            "https://api.github.com/repos/opennomad/systab/readme" \
             | grep -oP 'https://camo\.githubusercontent\.com/[^"]+' \
             | sort -u \
             | xargs -r -I{} curl -sX PURGE '{}' || true