It'll depend on the player. If Stephens insists that he wants to go to [insert club name here] and jumps up and down (publicly or in private) to say so, he will probably go. If he does not insist he wants to go and is happy to see out his contract, then he'll stay unless Brighton receive an...
When a player is out of contract in summer, he can negotiate with any other club from January. I think he can sign a pre-contract as well, but if someone knows different I'm willing to believe them.
The agent may be right that Stephens would get more money by waiting until he is out of...