Removes the assignment and detaches the template from the account's sleeves: they stay on the account with their strategies and targets and are no longer template-managed. If accounts gives no clear answer the unassign is kept, still unassigning, and the response is 202 with the assignment; reading it by id or sending the delete again finishes it. A delete that finds an earlier create or edit still being applied answers 409 change_in_flight, without recording the unassign.