During his confirmation last year, Attorney General Eric H. Holder Jr. failed to notify the Senate that he had signed several briefs urging courts to reject President George W. Bush’s claim that he had the power to imprison an American citizen as an “enemy combatant,” the Justice Department acknowledged Thursday. The briefs should have been disclosed as part of the confirmation process,” said Matthew Miller, a Justice Department spokesman. “In preparing thousands of pages for submission, it was unfortunately and inadvertently missed. In any event, the attorney general has publicly discussed his positions on detention policy on many occasions, including...