feat(slack): isolate session context per thread

Each Slack thread now gets its own conversation session instead of
sharing one session per channel. DM sessions are unchanged.

Added as a generic feature to also support if Feishu threads support
is added in the future.

nanobot/channels/base.py

@@ -111,13 +111,15 @@ class BaseChannel(ABC):
             )
             return
         
+        meta = metadata or {}
         msg = InboundMessage(
             channel=self.name,
             sender_id=str(sender_id),
             chat_id=str(chat_id),
             content=content,
             media=media or [],
-            metadata=metadata or {}
+            metadata=meta,
+            session_key_override=meta.get("session_key"),
         )
         
         await self.bus.publish_inbound(msg)

nanobot/channels/slack.py

@@ -179,6 +179,9 @@ class SlackChannel(BaseChannel):
         except Exception as e:
             logger.debug("Slack reactions_add failed: {}", e)
 
+        # Thread-scoped session key for channel/group messages
+        session_key = f"slack:{chat_id}:{thread_ts}" if thread_ts and channel_type != "im" else None
+
         try:
             await self._handle_message(
                 sender_id=sender_id,
@@ -189,7 +192,8 @@ class SlackChannel(BaseChannel):
                         "event": event,
                         "thread_ts": thread_ts,
                         "channel_type": channel_type,
-                    }
+                    },
+                    "session_key": session_key,
                 },
             )
         except Exception: