feat: add Go config validation tests

[jflowers]

Description

Add Go test files that validate the org/config.yaml and cross-tool consistency between Peribolos and safe-settings.

Acceptance Criteria

• config/config_test.go created:
  • Module: github.com/unbound-force/.github
  • Parses org/config.yaml using prow types
  • Validates CODEOWNERS users are org admins
  • Checks for duplicate admin/member entries
  • Validates sorted lists (admins, members, team members)
  • Validates team privacy is "closed"
  • Validates team membership rules (maintainers are admins, members are org members)
• config/boundary_test.go created:
  • Validates all suborg repos exist in peribolos repos: section
  • Checks no repo appears in multiple suborg files
  • Validates safe-settings doesn't set fields owned by peribolos (description, has_projects, default_branch)
  • Validates peribolos doesn't set fields owned by safe-settings
  • Validates suborg repo lists match ruleset repository_name conditions
• All tests pass: go test ./...

References

• Modeled on: zero-dot-force/.github/config/config_test.go and config/boundary_test.go