[Feature]: Notify agent of GITHUB_TOKEN when provided

[neubig]

What problem or use case are you trying to solve?

When using the UI and accessing a github repository, it is standard for OpenHands to use GITHUB_TOKEN. But it doesn't necessarily know that GITHUB_TOKEN is available. Because of this, it may suggest alternative, non-optimal solutions such as adding an ssh key.

Describe the UX of the solution you'd like

It would be nice if the agent knew that it should use GITHUB_TOKEN by default.

Do you have thoughts on the technical implementation?

We may want to consider prompting the agent and telling it that it has access to the GITHUB_TOKEN environment variable when it does, although I'm not sure if this should be implemented through the frontend or backend.