refactor(providers): hide oauth-only providers from config setup

Exclude openai_codex alongside github_copilot from generated config,
filter OAuth-only providers out of the onboarding wizard, and clarify in
README that OAuth login stores session state outside config. Also unify
the GitHub Copilot login command spelling and add regression tests.

Made-with: Cursor

tests/test_onboard_logic.py

@@ -359,6 +359,8 @@ class TestProviderChannelInfo:
         assert len(names) > 0
         # Should include common providers
         assert "openai" in names or "anthropic" in names
+        assert "openai_codex" not in names
+        assert "github_copilot" not in names
 
     def test_get_channel_names_returns_dict(self):
         from nanobot.cli.onboard_wizard import _get_channel_names