Show login options when not signed in with ChatGPT (#2440)

Motivation: we have users who uses their API key although they want to
use ChatGPT account. We want to give them the chance to always login
with their account.

This PR displays login options when the user is not signed in with
ChatGPT. Even if you have set an OpenAI API key as an environment
variable, you will still be prompted to log in with ChatGPT.

We’ve also added a new flag, `always_use_api_key_signing` false by
default, which ensures you are never asked to log in with ChatGPT and
always defaults to using your API key.



https://github.com/user-attachments/assets/b61ebfa9-3c5e-4ab7-bf94-395c23a0e0af

After ChatGPT sign in:


https://github.com/user-attachments/assets/d58b366b-c46a-428f-a22f-2ac230f991c0

README.md

@@ -22,6 +22,7 @@
     - [Authenticate locally and copy your credentials to the "headless" machine](#authenticate-locally-and-copy-your-credentials-to-the-headless-machine)
     - [Connecting through VPS or remote](#connecting-through-vps-or-remote)
   - [Usage-based billing alternative: Use an OpenAI API key](#usage-based-billing-alternative-use-an-openai-api-key)
+    - [Forcing a specific auth method (advanced)](#forcing-a-specific-auth-method-advanced)
   - [Choosing Codex's level of autonomy](#choosing-codexs-level-of-autonomy)
     - [**1. Read/write**](#1-readwrite)
     - [**2. Read-only**](#2-read-only)
@@ -165,6 +166,35 @@ Notes:
 - This command only sets the key for your current terminal session, which we recommend. To set it for all future sessions, you can also add the `export` line to your shell's configuration file (e.g., `~/.zshrc`).
 - If you have signed in with ChatGPT, Codex will default to using your ChatGPT credits. If you wish to use your API key, use the `/logout` command to clear your ChatGPT authentication.
 
+#### Forcing a specific auth method (advanced)
+
+You can explicitly choose which authentication Codex should prefer when both are available.
+
+- To always use your API key (even when ChatGPT auth exists), set:
+
+```toml
+# ~/.codex/config.toml
+preferred_auth_method = "apikey"
+```
+
+Or override ad-hoc via CLI:
+
+```bash
+codex --config preferred_auth_method="apikey"
+```
+
+- To prefer ChatGPT auth (default), set:
+
+```toml
+# ~/.codex/config.toml
+preferred_auth_method = "chatgpt"
+```
+
+Notes:
+
+- When `preferred_auth_method = "apikey"` and an API key is available, the login screen is skipped.
+- When `preferred_auth_method = "chatgpt"` (default), Codex prefers ChatGPT auth if present; if only an API key is present, it will use the API key. Certain account types may also require API-key mode.
+
 ### Choosing Codex's level of autonomy
 
 We always recommend running Codex in its default sandbox that gives you strong guardrails around what the agent can do. The default sandbox prevents it from editing files outside its workspace, or from accessing the network.