Add explicit least-privilege permissions to build workflow

[Copilot]

This addresses the code scanning finding on the GitHub Actions workflow by making GITHUB_TOKEN permissions explicit. The build workflow now declares only the repository access it requires.

• Workflow permissions

  • Add a top-level permissions block to /home/runner/work/dd-plist/dd-plist/.github/workflows/build.yml
  • Scope the workflow token to read-only repository contents

• Behavioral impact

  • No job logic, build steps, or artifact handling changed
  • The change is limited to workflow security posture

permissions:
  contents: read