Great idea!
I created a human-based service to do something similar.
The key problem with using AI to do this is that AI might not be able to deliver the feeling of being held accountable to some people. A huge part of having a an accountability partner is having somebody there that you don't want to disappoint.
However, I can say that services like woebot are pretty dang convincing. I would encourage you to play around with their app to see how they engineered their conversation flows to seem friendly and unobtrusive.