The test workflow was hitting GitHub's unauthenticated API rate
limit (60 req/hour) when downloading 95 packages via composer
install --prefer-dist, causing 429 Too Many Requests errors.
Two fixes applied:
1. Cache Composer's download cache (~/.composer/cache) keyed on
composer.lock hash, so repeated CI runs don't re-download
packages at all.
2. Configure GitHub OAuth token via SYNC_GITHUB_TOKEN secret to
raise the rate limit to 5,000 req/hour for cache misses.
lyra