WASHINGTON — Looking back, John C. Kiriakou admits he should have known better. But when the F.B.I. called him a year ago and invited him to stop by and “help us with a case,” he did not hesitate. Only an hour into what began as a relaxed chat with the two agents — the younger one who traded Pittsburgh Steelers talk with him and the senior investigator with the droopy eye — did he begin to realize just who was the target of their investigation. On Jan. 25, Mr. Kiriakou is scheduled to be sentenced to 30 months in prison...