the govern- ment to adhere to the terms of any plea bargain or immunity agreement it makes. Ste Mabry v. Johnson, 467 U.S. 504, 104 S.Ct. 2643, 81 L.Ed.2t1 437 (1984) (plea agreement); Santobetto v. New York, 404 (the date of the immunity agreement). or any other legal action, such as forfeiture. that