Improve the UX of the login flow.

While https://github.com/openai/codex/issues/1243 is about ensuring the _correctness_ of the login flow, this issue is about improving the UX of the login flow. https://github.com/openai/codex/pull/1212 touches on this, but ideally:

• The user should not have to run a command outside the TUI in order to log in. (Currently, they have to run codex login separately.) This is a little tricky because it requires defining a message protocol between the Rust code and login_with_chatgpt.py so we can surface the right information in the TUI (such as the URL the user has to open to go through the login flow, assuming it didn't open automatically for them).
• Ideally, the login flow would not require running a local webserver, but instead would be more like GitHub's flow where you enter a code on github.com. With the current setup, it is not possible for the user to complete the flow in a headless environment (e.g., a Docker container without port forwarding enabled on 1455).

Note that, while we should keep the user in the product for the login flow, we should probably continue to support the codex login subcommand.