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/pairing/store.py

@@ -82,6 +82,8 @@ def generate_code(
     with _LOCK:
         data = _load()
         _gc_pending(data)
+        # Collision probability is negligible (~1e-12 with 20 pending codes),
+        # so we skip an existence check for simplicity.
         raw = "".join(secrets.choice(_ALPHABET) for _ in range(_CODE_LENGTH))
         code = f"{raw[:4]}-{raw[4:]}"