Broken author links removed for direct-push stories

Direct-push commits previously constructed GitHub profile URLs from git committer names, which produced dead links. Display names like "Nathan Zylbersztejn" don't map to GitHub handles, so the resulting URLs returned 404s. The fix removes the fallback URL entirely—direct-push author bylines now render as plain text. The StoryCard UI already had a fallback path for missing URLs, so no interface changes were needed. PR-merge stories continue to link correctly using verified author URLs from GraphQL data.