Actually she is not a golden dragon, her father is. She got the ability to polymorph from him, but she can only assume one human form, unlike a real golden dragon. Anyway, it's not that important, just wanted to point that out.

Who did try to kill her?

And about that dagger. Why didn't Geralt jam it into her chest like Phillipa said to?

I believe there is a quest that you can find out who actually made the attempt but you can assume who the real mind behind the matter is.

I'm assuming Geralt didn't stab her with the dagger because he knew better than to trust every word the sorceress told him. She was already impaled and perhaps only the blood was enough.

Like I said though, just conjecture.