refactor(channels): formalize default config onboarding

tests/test_channel_default_config.py

@@ -0,0 +1,9 @@
+from nanobot.channels.registry import discover_channel_names, load_channel_class
+
+
+def test_builtin_channels_expose_default_config_dicts() -> None:
+    for module_name in sorted(discover_channel_names()):
+        channel_cls = load_channel_class(module_name)
+        payload = channel_cls.default_config()
+        assert isinstance(payload, dict), module_name
+        assert "enabled" in payload, module_name