build: produce universal macOS binary instead of separate arch builds

[bojanrajkovic]

Summary

• Add GoReleaser universal_binaries section with replace: true to combine darwin/amd64 and darwin/arm64 into a single Mach-O universal binary
• One macOS download works on both Intel and Apple Silicon
• Archive named cpm_<version>_darwin_universal.tar.gz instead of separate darwin_amd64 and darwin_arm64
• Homebrew cask auto-generates a single on_macos block instead of split on_intel/on_arm

🤖 Generated with Claude Code

[bojanrajkovic]

Test Plan

• goreleaser check validates config (only pre-existing deprecation warning about homebrew_casks.binary)
• goreleaser build --snapshot --clean produces dist/cpm-universal_darwin_all/cpm as a Mach-O universal binary with both x86_64 and arm64
• goreleaser release --snapshot --clean --skip=publish produces correct archive names:
  • cpm_<ver>_darwin_universal.tar.gz (single macOS archive)
  • cpm_<ver>_linux_amd64.tar.gz (unchanged)
  • cpm_<ver>_linux_arm64.tar.gz (unchanged)
  • cpm_<ver>_windows_amd64.zip (unchanged)
• Generated homebrew cask (dist/homebrew/Casks/cpm.rb) has single on_macos block pointing to darwin_universal.tar.gz
• No individual darwin_amd64 or darwin_arm64 archives are produced (confirmed by replace: true)