Mentors onboarding github details

After adding people's github handles, attach to specific repos. Learned this when one Mentor couldn't assign anyone as `assignee` (didn't have the settings gear icon)

<img width="812" alt="Image" src="https://github.com/user-attachments/assets/5446656d-4aa2-4952-9476-ba8c5a9a37f1" />