Support ChatGPT subscription or Codex CLI for AI assistance

### **Feature summary**

Add support for integrating external ChatGPT subscriptions or Codex CLI tools to enhance AI capabilities.

### **What problem are you trying to solve?**

Users are currently limited in how they can leverage AI assistance within the app. Developers who already pay for external AI services (like ChatGPT Plus or OpenAI/Codex APIs) cannot utilize their existing subscriptions, forcing them to constantly context-switch between the app, external terminal windows, or browser tabs.

### **Proposed solution**

Introduce a setting or configuration layer that allows users to connect their own AI provider. Specifically, add support for either:

1. Authenticating via a **ChatGPT subscription** (allowing users to leverage their existing paid tier).
2. Integrating with **GitHub Copilot / Codex CLI**.

This would allow the GitHub App to leverage these external endpoints directly within its workflow.

### **Workflow impact**

This impacts developers who want deep AI integration but prefer (or already pay for) ecosystems outside the default setup. It significantly improves the workflow by eliminating context-switching, reducing friction, and giving users the flexibility to bring their own tooling.

### **Installation context**

*Leave blank or use:* Organization and user-level installations.

### **Additional context**

A great reference for how a ChatGPT subscription integration can be seamlessly handled in a developer tool is Zed's implementation:
👉 [https://zed.dev/blog/chatgpt-subscription-in-zed]()