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/bus/events.py

@@ -16,11 +16,12 @@ class InboundMessage:
     timestamp: datetime = field(default_factory=datetime.now)
     media: list[str] = field(default_factory=list)  # Media URLs
     metadata: dict[str, Any] = field(default_factory=dict)  # Channel-specific data
+    session_key_override: str | None = None  # Optional override for thread-scoped sessions
     
     @property
     def session_key(self) -> str:
         """Unique key for session identification."""
-        return f"{self.channel}:{self.chat_id}"
+        return self.session_key_override or f"{self.channel}:{self.chat_id}"
 
 
 @dataclass