refactor(cli): remove OpenAI codex auth plugin setup

OpenCode now natively supports OpenAI authentication, so we no longer
need to:
- Add opencode-openai-codex-auth plugin
- Configure CODEX_PROVIDER_CONFIG provider settings

The --chatgpt flag still controls Oracle agent model selection
(GPT-5.2 vs fallback).

README.md

@@ -94,8 +94,7 @@
       - [Google Gemini (Antigravity OAuth)](#google-gemini-antigravity-oauth)
         - [Model Configuration](#model-configuration)
         - [oh-my-opencode Agent Model Override](#oh-my-opencode-agent-model-override)
-      - [OpenAI (ChatGPT Plus/Pro)](#openai-chatgpt-pluspro)
-        - [Model Configuration](#model-configuration-1)
+
     - [⚠️ Warning](#️-warning)
     - [Verify the setup](#verify-the-setup)
     - [Say 'Congratulations! 🎉' to the user](#say-congratulations--to-the-user)