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_commands.py

@@ -213,6 +213,15 @@ def test_config_matches_openai_codex_with_hyphen_prefix():
     assert config.get_provider_name() == "openai_codex"
 
 
+def test_config_dump_excludes_oauth_provider_blocks():
+    config = Config()
+
+    providers = config.model_dump(by_alias=True)["providers"]
+
+    assert "openaiCodex" not in providers
+    assert "githubCopilot" not in providers
+
+
 def test_config_matches_explicit_ollama_prefix_without_api_key():
     config = Config()
     config.agents.defaults.model = "ollama/llama3.2"