I won't scold you. It's obviously not necessary when you've long since taken care of that yourself. Not sure why others feel the need to do that, either.

You don't have to be read your Miranda rights unless you're being held, either arrested or just held "custodially," so you can be asked questions relating to the commission of a crime. You weren't arrested, held, or questioned. Just summoned to a court date because of the charge you received, so you didn't have to be Mirandized. It's a common misconception that everyone under all circumstances of arrest or detention must be Mirandized in the United States. Miranda rights have to do with questioning about a crime.
Miranda warning - Wikipedia, the free encyclopedia

I suppose that anything you said could be used against you in court, but it's unlikely. Of course, I don't know what you said or admitted to. But what'll be used against you in this court proceeding is what the undercover cops witnessed.