Function spawn_in_worktree 

pub fn spawn_in_worktree(
    worktree_path: &Path,
    prompt: Option<&str>,
) -> Result<()>

Launch the configured AI tool inside an existing worktree.

Used by both gw new (after worktree creation) and gw spawn. Honors the default launch method from config; no terminal-override / fg / bg knobs.