feat: able to send to multiple receivers

ref: N25B-441

src/control_backend/core/agent_system.py

@@ -130,7 +130,12 @@ class BaseAgent(ABC):
 
         :param message: The message to send.
         """
-        target = AgentDirectory.get(message.to)
+        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.")

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