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/internal_message.py

@@ -0,0 +1,12 @@
+from pydantic import BaseModel
+
+
+class InternalMessage(BaseModel):
+    """
+    Represents a message to an agent.
+    """
+
+    to: str
+    sender: str
+    body: str
+    thread: str | None = None