Function lookup_registry_skill_tier 

pub fn lookup_registry_skill_tier(
    registry_dir: &Path,
    name: &str,
) -> (SkillTier, Option<String>)

Look up a skill in <registry_dir>/registry.json and return its trust tier and version. Returns (SkillTier::Unknown, None) if the index file is missing, malformed, or does not list the skill.