I am a 2013 Project Server user. As far as I know, there are no workflow capabilities at the task level.
If you’re looking for workflows that can track approval points and so on, I think you’re out of luck.
If you’re just trying to understand how work is progressing and you’re using very large, more generic tasks, might I suggest smaller tasks, dependency relationships, and time tracking at the task level. Its not automated, in a way a workflow could be, but it does allow tracking and reporting capabilities of progress between tasks (stages?).
Hope that helps