I personally don't think cortana is truly dead. Think about it, if you are an ai, your mind functions close to the speed of light. She could have copied her mind, ditched the sexy shell, compressed the file, and stored it in chief's suit system data, just before deletion. I believe the armor suits can house up to four ai, but the user usually mentally and physically snaps with two. It would take an extremely disciplined mind to withstand four So thus there would be room for a "sleeping" copy