ci: add GitHub Actions platform demos

[quanruzhuoxiu]

Summary

• add GitHub Actions demo workflows for Windows, macOS, Android emulator, and iOS simulator examples
• run the existing Midscene example smoke tests and upload generated Midscene reports as artifacts
• keep these reusable integration examples in midscene-example instead of copying Midscene monorepo workflow internals

Validation

• ruby -e 'require "yaml"; ARGV.each { |f| YAML.load_file(f); puts "ok #{File.basename(f)}" }' .github/workflows/windows-computer-demo.yaml .github/workflows/macos-computer-demo.yaml .github/workflows/android-emulator-demo.yaml .github/workflows/ios-simulator-demo.yaml