Function setup_fork_remote 

pub fn setup_fork_remote(
    repo: &Repository,
    metadata: &PrMetadata,
) -> Result<String>

Ensure a remote for a fork PR exists, then return its name.

For non-fork PRs this is equivalent to detect_pr_remote. For fork PRs, a remote named pr-{number}-fork is added if it doesn’t already exist, pointing at the fork’s clone URL.