BUJUMBURA, Burundi — In the hours after rebel forces launched a pre-dawn assault on three military installations here on Friday, the embattled government claimed to have killed 79 insurgents in a joint military and police security operation. Some insurgents died in the initial clashes with the military, the government said, while others supposedly retreated into neighborhoods known for their staunch opposition to the regime, where they were later shot by security forces and left bleeding in the streets.