Add a way for Codex diff to be saved or exported

What version of the VS Code extension are you using?

0.4.56

What subscription do you have?

Plus

Which IDE are you using?

VS Code

What platform is your computer?

Darwin 25.2.0 arm64 arm

What issue are you seeing?

Codex Diff produced by edits can be opened by clicking the files modified when codex makes a change, however it cannot be exported to a .patch or otherwise saved:

<img width="1031" height="678" alt="Image" src="https://github.com/user-attachments/assets/79626b1a-5241-4124-bbb5-30f4c5260356" />

This is rather frustrating when you want to selectively undo/redo changes made at an earlier point in the conversation.

What steps can reproduce the bug?

1. Ask for changes
2. Click on the modified files (a Codex Diff window openens)

What is the expected behavior?

File > Save As or a button would permit exporting the changes as a standard .patch diff file

Additional information

_No response_