Main BDI brain structure implemented. Still some TODOs left, and very
basic implementation (only one belief "user_said(Message)" and every
message is sent straight to a function which is responsible for getting
an LLM response.
ref: N25B-197
Inside the `/message` enpoint we put a message onto the internal event
queue, which gets read by TestAgent. This agent, in turn, logs the
message (temporary behaviour until we implement RI integration).
The name TestAgent is of course temporary, this is just for exploratory
purposes.
ref: N25B-165
