feat: pydantic models and inter-process messaging

Moved `InternalMessage` into schemas and created a `BeliefMessage`
model. Also added the ability for agents to communicate via ZMQ to
agents on another process.

ref: N25B-316

src/control_backend/schemas/belief_message.py

@@ -0,0 +1,5 @@
+from pydantic import BaseModel
+
+
+class BeliefMessage(BaseModel):
+    beliefs: dict[str, list[str]]