WASHINGTON — A divided federal appeals court on Friday threw out an agreement that would have allowed accused Sept. 11 mastermind Khalid Sheikh Mohammed to plead guilty in a deal sparing him the risk of execution for al-Qaida’s 2001 attacks. The decision by a panel of the federal appeals court in Washington, D.C., undoes an attempt to wrap up more than two decades of military prosecution beset by legal and logistical troubles. It signals there will be no quick end to the long struggle by the U.