docs: add docs to CB

Pretty much every class and method should have documentation now.

ref: N25B-295

src/control_backend/schemas/internal_message.py

@@ -3,7 +3,12 @@ from pydantic import BaseModel
 
 class InternalMessage(BaseModel):
     """
-    Represents a message to an agent.
+    Standard message envelope for communication between agents within the Control Backend.
+
+    :ivar to: The name of the destination agent.
+    :ivar sender: The name of the sending agent.
+    :ivar body: The string payload (often a JSON-serialized model).
+    :ivar thread: An optional thread identifier/topic to categorize the message (e.g., 'beliefs').
     """
 
     to: str