SHEAR YASHUV, Israel — The Lebanese militant Hezbollah group fired a salvo of missiles at an Israeli military convoy in a disputed border area Wednesday, killing two soldiers and triggering deadly clashes that marked the most serious escalation since the sides’ 2006 war. The flare-up, which left a U.N. peacekeeper dead and seven Israeli soldiers wounded, adds to the regional chaos brought on by neighboring Syria’s civil war.