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/agents/bdi/belief_collector_agent.py

@@ -3,6 +3,7 @@ import json
 from control_backend.agents.base import BaseAgent
 from control_backend.core.agent_system import InternalMessage
 from control_backend.core.config import settings
+from control_backend.schemas.belief_message import BeliefMessage
 
 
 class BDIBeliefCollectorAgent(BaseAgent):
@@ -80,7 +81,7 @@ class BDIBeliefCollectorAgent(BaseAgent):
         msg = InternalMessage(
             to=settings.agent_settings.bdi_core_name,
             sender=self.name,
-            body=json.dumps(beliefs),
+            body=BeliefMessage(beliefs=beliefs).model_dump_json(),
             thread="beliefs",
         )