Cortana wouldnt be able to hack ghost, ghost is centuries ahead in technology, and can move about on its own. Sure she can hack covenant and forerunner technology, but can she hack sentient light-based mechanical lifeforms? No. Ghost however, could easily shut down cortana and master chiefs suit, putting him in a sleeping state where any kinderguardian could walk up and disintegrate chief with a slap. But guardians don't play cheap like that, we would fight chief head on, and definitely win.