Function create_worktree 

pub async fn create_worktree(
    repo_dir: &Path,
    issue_number: u32,
    base_branch: &str,
) -> Result<Worktree>

Create a worktree for the given issue, branching from origin/<base_branch>.

Uses the remote tracking ref rather than the local branch so that worktrees always start from the latest remote state (e.g. after PRs are merged).