An installation account is bound to a single user or organization, so the account's username is always the repository owner, and repository visibility doesn't affect access. The new plan asks only for the repository name. `check_repository()` gets an app-specific not-found message and skips the public/private mismatch validation, which doesn't apply to app access. The owner lookups treat the plan like the personal plans, resolving to the account username. The form UI doesn't offer this plan yet; that's wired up separately by filtering plans based on the selected account. Testing Done: Ran unit tests.