Skip unsupported windows/arm release asset build

[Copilot]

The release GitHub Actions job was failing in the asset build step because the release matrix included windows/arm, which is not a supported Go target. This change narrows the failure to the unsupported combination without affecting valid release artifacts.

• Release asset target handling

  • Add an explicit release-assets/windows.arm rule in Makefile
  • Treat windows/arm the same way darwin/arm is already handled: skip it as unsupported

• Behavioral impact

  • Preserve all currently supported release assets
  • Avoid failing the entire release workflow on one invalid GOOS/GOARCH pair

release-assets/darwin.arm:
	echo "not supported"

release-assets/windows.arm:
	echo "not supported"