Create an enhancement/design for rewrite

[shawn-hurley]

As a first step, we need to design and diagram how we expect to build the agents moving forward, more clearly delineating the api boundaries, and explaining the graphs and how to lay out nodes and edges so they are more easily shared.

We also need to document how we think about checkpointing and the persistence of the state.

We should also make sure that whatever we do, is able to fit into the #674 rewrite to accommodate #153.