Nicaragua extended a seven-month-old cease-fire with U.S.-backed Contras until the end of November despite what it said was continued aggression by the rebels and their U.S. sponsors. “The government . . . has decided to prolong unilaterally until Nov. 30 of the present year a halt to offensive actions,” a statement from President Daniel Ortega said. The cease-fire, which took effect April 1, had been due to run out at midnight Monday. The Contras have said they will respect the truce indefinitely but accuse the leftist Sandinista government of constantly violating it.