Clarify GitHub auth scope when personal and work accounts overlap

[untitaker]

The GitHub auth permission prompt doesn't explain that Junior's access is scoped to repos where the GitHub App is installed. Users connecting their GitHub account may not realize this means Junior can only act on repos with the App explicitly installed — not their entire account.

Current behavior

• Junior operates using the GitHub App installation scope; write access requires the App to be installed on the target repo
• The permission prompt doesn't state this clearly

Gap
Users don't know from the prompt alone that "connect GitHub" means "repos where the Junior App is installed." A clearer explanation would set accurate expectations upfront.

There is also an edge case worth documenting: if a user has the Junior App installed on multiple orgs, Junior's credential could reach repos across those orgs. In practice this is unlikely — Junior currently only runs for getsentry — but worth noting in the prompt or docs.

Suggested improvement
Clarify in the GitHub auth prompt or onboarding that Junior's access is limited to repos where the GitHub App is installed, and point users to github.com/settings/installations to review their App installations.