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/config.md

@@ -612,5 +612,4 @@ Options that are specific to the TUI.
 | `experimental_use_exec_command_tool` | boolean | Use experimental exec command tool. |
 | `responses_originator_header_internal_override` | string | Override `originator` header value. |
 | `projects.<path>.trust_level` | string | Mark project/worktree as trusted (only `"trusted"` is recognized). |
-| `preferred_auth_method` | `chatgpt` \| `apikey` | Select default auth method (default: `chatgpt`). |
 | `tools.web_search` | boolean | Enable web search tool (alias: `web_search_request`) (default: false). |