expose agent system prompt + give it chat functionality

[AlephNotation]

No description provided.

[AlephNotation]

Ok got a bit carried away here since we're introducing breaking changes.

This PR now includes an overhaul of the model's response typing. We no longer need to extend ModelResponseType and hope that it includes the extended optional type in the payload. I know this may seem overkill but I was running into this problem yesterday. Plus the more we can enforce type strictness the better these things seem to do.

We now perform a merge with ObjectiveComplete so that if a model wants to return an ObjectiveComplete response it must also include any additional fields we've defined.

[matildepark]

I don't like it when things work, so let's just do it