Families had waited three months for a break. Before dawn today, Mexico City authorities said they finally had one: 13 people detained in the killing of two senior government staffers whose daylight slaying jarred the capital in May. Officials said three of the detainees allegedly executed the attack; the rest helped plan and support it. The City’s security cabinet framed the case as a coordinated hit by a cell that has now been dismantled. 13 ar…