dsh-openai-codex-oauth: Add ChatGPT subscription OAuth login for DeepSeek Harness's openai-codex provider

`dsh-openai-codex-oauth` is a community plugin for DeepSeek Harness, primarily addressing the issues of ChatGPT subscription OAuth login, credential management, and token refresh for the `openai-codex` provider within the Web interface. This plugin is released under the MIT license, supporting interactive login (`/codex-login`), status queries (`/codex-status`), and logout (`/codex-logout`), while being compatible with browser OAuth (PKCE) and device code login methods. Credentials are persistently stored in specified environment variables to ensure automatic access token refresh. Installation requires DeepSeek Harness 0.1.0-rc.6 and Node.js 22.19.