Function held_elsewhere_error 

pub(super) fn held_elsewhere_error(other: &str, closing: bool) -> String

The one spawn refusal that is a decision, not a failure: --resume <sid> was asked for while another pane still holds (or is still closing) that same conversation — session_held_elsewhere. Built here so that is_held_elsewhere_error is checked against the same text spawn_process actually returns, rather than a second copy that could drift; tests/reopen_guard.rs pins the wording users see.