JOHANNESBURG, March 30 — Top military commanders from the Angolan government and the nation's Unita rebel movement signed a preliminary cease-fire agreement this afternoon, moving the country an important step closer to ending a civil war that defied many previous attempts at peace. The agreement, which is expected be formally endorsed in the coming week in the Angolan capital, Luanda, comes barely a month after Jonas Savimbi, the longtime leader of the rebel National Union for the Total Independence of Angola, or Unita, was killed in an ambush by government commandos. After initially continuing its offensive, the government unexpectedly halted...