Simplify auth flow and reconcile differences between ChatGPT and API Key auth (#3189)

This PR does the following:
* Adds the ability to paste or type an API key.
* Removes the `preferred_auth_method` config option. The last login
method is always persisted in auth.json, so this isn't needed.
* If OPENAI_API_KEY env variable is defined, the value is used to
prepopulate the new UI. The env variable is otherwise ignored by the
CLI.
* Adds a new MCP server entry point "login_api_key" so we can implement
this same API key behavior for the VS Code extension.
<img width="473" height="140" alt="Screenshot 2025-09-04 at 3 51 04 PM"
src="https://github.com/user-attachments/assets/c11bbd5b-8a4d-4d71-90fd-34130460f9d9"
/>
<img width="726" height="254" alt="Screenshot 2025-09-04 at 3 51 32 PM"
src="https://github.com/user-attachments/assets/6cc76b34-309a-4387-acbc-15ee5c756db9"
/>

docs/advanced.md

@@ -8,7 +8,7 @@ Run Codex head-less in pipelines. Example GitHub Action step:
 - name: Update changelog via Codex
   run: |
     npm install -g @openai/codex
-    export OPENAI_API_KEY="${{ secrets.OPENAI_KEY }}"
+    codex login --api-key "${{ secrets.OPENAI_KEY }}"
     codex exec --full-auto "update CHANGELOG for next release"
 ```