[Feature]: Allow selecting branch and/or PR for each repo

[xingyaoww]

What problem or use case are you trying to solve?

For task templates like this:
https://github.com/All-Hands-AI/OpenHands/blob/main/microagents/tasks/update_pr_description.md

I will have to fill in the PR URL and branch name manually.

Describe the UX of the solution you'd like

It would be nice if we could optionally select the branch to checkout to after picking the GH repo, and the agent is automatically aware of that information (eg "you are checked out to branch "XXX", which belongs to PR "XXX")

Do you have thoughts on the technical implementation?

Describe alternatives you've considered

Additional context