Introduce equivalent of Copilot's `#githubRepo` context

What feature would you like to see?

I would like the Codex VS Code extension to support a repository context experience similar to the Copilot Chat extension's #githubRepo implementation:

<img width="911" height="298" alt="Image" src="https://github.com/user-attachments/assets/329c881d-f827-4103-ac1e-4f4aefac8af3" />

Currently, when providing a GitHub repository URL as context, I need to manually approve a large number of individual files one by one. This becomes time-consuming and makes it difficult to efficiently work with full-repository context.

<img width="408" height="203" alt="Image" src="https://github.com/user-attachments/assets/8f8a6de7-4bcc-4dfa-84b0-55eb791e3a27" />

Although Agent (full access) mode can be used as a workaround, it introduces broader access than may be desirable and still requires the system to process files individually, which feels inefficient for this use case.

A more streamlined, repo-level context mechanism (similar to Copilot's approach) would significantly improve usability and workflow efficiency when working with larger codebases.

Additional information

_No response_