Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/repo/repo.go

@@ -878,7 +878,7 @@ func IsRepositoryModelExist(ctx context.Context, u *user_model.User, repoName st
 // non-generated repositories, and TemplateRepo will be left untouched)
 func GetTemplateRepo(ctx context.Context, repo *Repository) (*Repository, error) {
 	if !repo.IsGenerated() {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil for non-generated repositories
 	}
 
 	return GetRepositoryByID(ctx, repo.TemplateID)