ci: resolve the Shopware version with Composer from the plugin constraint; harden the registry repack

- phpunit: 'Resolve the latest Shopware Version' uses composer show against require.shopware/core
  from the plugin's composer.json (the Packagist scrape returned nothing on the first run)
- release: build the registry zip from an explicit file list without a leading ./ so the
  composer.json entry is stored at the zip root

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -126,7 +126,8 @@ jobs:
           fi
           test -f "$ROOT/composer.json" || { echo "::error::no composer.json found in the packaged zip"; exit 1; }
           OUT="$PWD/.build/composer-${{ steps.meta.outputs.zip }}"
-          (cd "$ROOT" && zip -qr "$OUT" .)
+          # explicit file list with the leading "./" stripped, so entries are stored as "composer.json", "src/..."
+          (cd "$ROOT" && find . -mindepth 1 | sed 's|^\./||' | zip -q "$OUT" -@)
           unzip -l "$OUT" | grep -E ' composer\.json$' > /dev/null || { echo "::error::repacked zip has no root-level composer.json"; exit 1; }
           echo "repacked: $OUT"