Where is oauth authentication?

If I run "gemini" from cli it should ask me the oauth authorization!
Instead this happens:

```
# npm install -g @google/gemini-cli

# gemini
Please set an Auth method in your /root/.gemini/settings.json or specify one of the following environment variables before running: GEMINI_API_KEY, GOOGLE_GENAI_USE_VERTEXAI, GOOGLE_GENAI_USE_GCA
```

I don't want to use the api key. I want to use Oauth2 as before.

Start by tracing the CLI authentication configuration that produces the settings.json and environment-variable message. Compare the current flow with the OAuth2 behavior described as previously available, then verify that running the installed gemini command prompts for OAuth authorization without requiring an API key.

Written by the indexing model from the issue text.