GUANTÁNAMO BAY, Cuba — In his first month in U.S. custody, the man accused of planning the 9/11 attacks confessed to the crime during questioning and wanted to keep talking about it, according to the psychologist who interrogated him. But the CIA wanted him to discuss al-Qaida’s future plans, not the attacks that had horrified America a year and a half earlier, said Dr. James Mitchell, the psychologist. So when the prisoner, Khalid Sheikh Mohammed, mentioned 9/11, they would slam him, naked, int