Function parse_owner_repo_ref 

pub fn parse_owner_repo_ref(input: &str) -> (String, Option<String>)

Parse owner/repo[@ref] into (owner_repo, ref_).

Supports:

• owner/repo → ("owner/repo", None)
• owner/repo@v4 → ("owner/repo", Some("v4"))
• owner/repo@main → ("owner/repo", Some("main"))