Add WindowsCmd transport that runs Playwright directly on Windows via
PowerShell stdin/stdout, completely bypassing WSL2 networking issues.
This eliminates the need for a running server and works around Hyper-V
firewall restrictions that block WebSocket connections.

Transport changes:
- Add Playwriter.Transport.WindowsCmd for direct Windows execution
- Disable Remote transport (WSL2 firewall blocks WebSockets)
- Add mode: :windows option to public API
- Update paths from deps/playwright_ex to deps/playwright/priv/static
- Fix screenshot handling with proper base64 decoding
- Increase GenServer call timeouts for reliability

Documentation additions:
- Add guides/testing.md with test categories, tags, and CI guidance
- Add examples/README.md documenting all examples and CLI flags
- Update mix.exs hexdocs with explicit filenames to avoid conflicts
- Group examples separately from guides in documentation

New examples:
- test_windows_cmd.exs for testing WindowsCmd transport directly
- windows_mode.exs demonstrating mode: :windows usage

Script and configuration updates:
- Add -ExecutionPolicy Bypass to all PowerShell commands
- Bind server to 0.0.0.0 for WSL2 accessibility
- Add screenshot.png to .gitignore
- Fix interaction.exs button selector
- Update support.exs with correct paths and instructions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>