feat: able to send to multiple receivers

ref: N25B-441

src/control_backend/core/agent_system.py

@@ -130,16 +130,21 @@ class BaseAgent(ABC):
 
         :param message: The message to send.
         """
-        target = AgentDirectory.get(message.to)
-        if target:
-            await target.inbox.put(message)
-            self.logger.debug(f"Sent message {message.body} to {message.to} via regular inbox.")
-        else:
-            # Apparently target agent is on a different process, send via ZMQ
-            topic = f"internal/{message.to}".encode()
-            body = message.model_dump_json().encode()
-            await self._internal_pub_socket.send_multipart([topic, body])
-            self.logger.debug(f"Sent message {message.body} to {message.to} via ZMQ.")
+        to = message.to
+        receivers = [to] if isinstance(to, str) else to
+
+        for receiver in receivers:
+            target = AgentDirectory.get(receiver)
+
+            if target:
+                await target.inbox.put(message)
+                self.logger.debug(f"Sent message {message.body} to {message.to} via regular inbox.")
+            else:
+                # Apparently target agent is on a different process, send via ZMQ
+                topic = f"internal/{message.to}".encode()
+                body = message.model_dump_json().encode()
+                await self._internal_pub_socket.send_multipart([topic, body])
+                self.logger.debug(f"Sent message {message.body} to {message.to} via ZMQ.")
 
     async def _process_inbox(self):
         """

src/control_backend/schemas/internal_message.py

@@ -1,3 +1,5 @@
+from collections.abc import Iterable
+
 from pydantic import BaseModel
 
 
@@ -11,7 +13,7 @@ class InternalMessage(BaseModel):
     :ivar thread: An optional thread identifier/topic to categorize the message (e.g., 'beliefs').
     """
 
-    to: str
+    to: str | Iterable[str]
     sender: str
     body: str
     thread: str | None = None