fix(agent): propagate effective session key through subagent pipeline

The previous fix hardcoded session_key_override as channel:chat_id which
broke unified session mode where pending queues use "unified:default".
Propagate the effective key from _set_tool_context through SpawnTool
into the origin dict so _announce_result routes to the correct pending
queue in both normal and unified session modes.

nanobot/agent/subagent.py

@@ -107,7 +107,7 @@ class SubagentManager:
         """Spawn a subagent to execute a task in the background."""
         task_id = str(uuid.uuid4())[:8]
         display_label = label or task[:30] + ("..." if len(task) > 30 else "")
-        origin = {"channel": origin_channel, "chat_id": origin_chat_id}
+        origin = {"channel": origin_channel, "chat_id": origin_chat_id, "session_key": session_key}
 
         status = SubagentStatus(
             task_id=task_id,
@@ -241,15 +241,17 @@ class SubagentManager:
         )
 
         # Inject as system message to trigger main agent.
-        # Use session_key_override to align with the main agent's session key
-        # so the result is routed to the pending queue (mid-turn injection)
-        # instead of being dispatched as a competing independent task.
+        # Use session_key_override to align with the main agent's effective
+        # session key (which accounts for unified sessions) so the result is
+        # routed to the correct pending queue (mid-turn injection) instead of
+        # being dispatched as a competing independent task.
+        override = origin.get("session_key") or f"{origin['channel']}:{origin['chat_id']}"
         msg = InboundMessage(
             channel="system",
             sender_id="subagent",
             chat_id=f"{origin['channel']}:{origin['chat_id']}",
             content=announce_content,
-            session_key_override=f"{origin['channel']}:{origin['chat_id']}",
+            session_key_override=override,
             metadata={
                 "injected_event": "subagent_result",
                 "subagent_task_id": task_id,