Allow Codex to access ChatGPT shared conversations

What issue are you seeing?

Codex is unable to access ChatGPT shared conversations.

<img width="2223" height="1041" alt="Image" src="https://github.com/user-attachments/assets/a1147583-3fa4-4d3a-aa55-3247f881caa5" />

What steps can reproduce the bug?

1. Go to a ChatGPT conversation and click on Share to get a sharable link (https://chatgpt.com/share/{id}).
2. Go to Codex and ask it if it can access that conversation

What is the expected behavior?

For Codex to be able to access ChatGPT conversations that are meant to be shared.

Additional information

I only tried it on Cloud Codex, but it probably also doesn't work on CLI, extension, app etc.