Function discard_worker_worktree 

pub fn discard_worker_worktree(
    repo: &Path,
    label: &str,
    path: &Path,
    ensured: WorktreeEnsure,
) -> Option<String>

Undo the work ensure_worker_worktree did for a spawn that was then refused, so the refusal strands neither worktree nor branch. The worktree goes only when ensure created it, the branch only when ensure minted it - an AttachedExistingBranch keeps the branch, which predates the call and may hold the worker’s only copy of its commits. Returns git’s complaint when something had to stay behind; the spawn is already refused, so a failure is a warning, not an error.