Function install_extra_registry_skill_source 

pub fn install_extra_registry_skill_source(
    source: &str,
    skills_path: &Path,
    allow_scripts: bool,
    workspace_dir: &Path,
    extra_registries: &[ExternalRegistry],
    suppress_tier_banner: bool,
) -> Result<(PathBuf, usize)>

Install a skill from a user-configured extra registry, addressed as registry:<name>/<skill>. The named registry must be present, enabled, and of kind = "git"; it reuses the same git-clone registry mechanism as the default bare-name registry and then installs the skill locally.