docs(pairing): address reviewer comments — comments, error msg, __all__ test

- Clarify SystemExit message for missing/null allowFrom (manager.py)
- Document why Feishu passes content="" for unauthorized DMs
- Document exact-match semantics in BaseChannel.is_allowed()
- Document negligible collision probability in generate_code()
- Add test_all_exports_are_importable for nanobot.pairing.__all__

nanobot/channels/base.py

@@ -202,6 +202,7 @@ class BaseChannel(ABC):
             allow_list = getattr(self.config, "allow_from", []) or []
         if "*" in allow_list:
             return True
+        # allowFrom entries are opaque tokens — must match exactly.
         if str(sender_id) in allow_list:
             return True
         if is_approved(self.name, str(sender_id)):