Function install_plugin_from_subdir 

pub fn install_plugin_from_subdir(
    marketplace_url: &str,
    subdir: &str,
    dest: &Path,
) -> Result<String, String>

Shallow-clone marketplace_url into a temp dir sibling to dest, then move its <subdir> directly into place at dest. Returns the HEAD SHA of the cloned marketplace. Used for Claude-Code-style marketplaces whose plugins reference ./<subdir> instead of their own standalone repos.

Guarantees:

• subdir must pass crate::skills::marketplace::is_safe_plugin_name (no traversal, no path separators).
• dest must not exist.
• If the subdir doesn’t exist inside the cloned repo, returns Err and does not create dest.