Agent Skills: librarian

Cache and refresh remote git repositories under `~/.cache/checkouts/<host>/<org>/<repo>` so future references reuse a local copy instead of re-cloning. Use when the user references a remote git repository — full URL (`https://github.com/foo/bar`, `git@github.com:foo/bar.git`), `owner/repo` shorthand, or asks Claude to "look at", "read", "search", or "find something in" an external repo by name. Returns a stable local path that downstream skills (review-code, grep, Read) can use directly.