Migrates lexicon schemas from social.coves.post.* to social.coves.community.post.* to better reflect atProto architecture where posts are records in community repositories. Changes: - Move post schemas to social.coves.community.post namespace - Update cross-references in feed/defs.json and embed/post.json - Update validation tool to handle defs-only files (skip #main validation) - Add skip logic for *.defs files in lexicon tests - Remove old social.coves.post/* schemas This aligns with atProto best practices for community-based content organization.