feat: add BDI core agent

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

src/control_backend/agents/bdi/rules.asl

@@ -0,0 +1,3 @@
++user_said(Message) : not responded <-
+  +responded;
+  .reply(Message).