Assuming the editor still has it, that's something to mention during your phone call with the agent (or maybe when they make a request) but not in the query. eg, "I've attached the requested manuscript for Awesome Title. Just so you know, an editor at RandomPenguin has it after requesting it in a contest, and if he responds positively, I won't accept an offer before I've notified you."
Congrats, and good luck!