Function create_retro_pr 

pub fn create_retro_pr(
    project_path: &str,
    files: &[(&str, &str)],
    commit_message: &str,
    pr_title: &str,
    pr_body: &str,
) -> Result<Option<String>, CoreError>

Create a retro PR: chdir → stash → branch from default → write files → commit → push → PR → restore. Returns the PR URL on success, or None if PR creation was skipped.