Add release workflow with tag/release existence checks

[kevinwang1975]

Summary

• CI workflow (ci.yml): replaced mvn -B package with mvn -B verify, removed artifact upload (releases handle distribution)
• New release workflow (release.yml): triggered manually via workflow_dispatch with a version input
  • Validates that neither the tag nor the release already exists before proceeding
  • Builds with Java 8
  • Pushes a v* tag
  • Creates a GitHub Release with the built JAR attached and auto-generated release notes