dsh-oauth-openai: OpenAI Codex OAuth Provider and Model Adapter (Retired)

`dsh-oauth-openai` is a deprecated plugin for DeepSeek Harness, used to handle OpenAI Codex OAuth authentication, and is now maintained only in maintenance mode. The official recommendation is to remove this plugin and the legacy Codex package to avoid conflicts. The relevant OAuth flow is now built into the main program’s `@deepseek-ai/dsh-llm-pi-ai` package.