TEL AVIV, Israel — An Israeli airstrike killed the chief spokesman for Lebanon’s Hezbollah militant group on Sunday in the first attack in central Beirut in more than a month, while Palestinian medical officials said earlier Israeli strikes killed 12 in Gaza. A Hezbollah official, speaking on the condition of anonymity, said Mohammed Afif was killed in the Beirut strike. He had been especially visible since Israel escalated its attacks in Lebanon in September. The action followed the assassination of longtime Hezbollah leader Hassan Nasrallah, who was also killed in an Israeli airstrike. People could be seen fleeing the neighborhood