Function prepare_pr_worktree 

pub fn prepare_pr_worktree(
    repo: &Repository,
    pr_number: u32,
    pr_format: &str,
) -> Result<(String, String, String)>

Prepare everything needed to create a worktree for PR pr_number.

Orchestrates the complete PR workflow:

1. Checks that gh CLI is available
2. Fetches PR metadata via gh
3. Sets up a fork remote if the PR is cross-repository
4. Fetches the PR’s head branch
5. Formats the worktree name using pr_format

Returns (worktree_name, remote_ref, base_branch) ready for add_worktree.