Skip CI when only Markdown changes

[SorraTheOrc]

Summary

• add a changes gate job that detects non-.md files
• skip the Playwright tests job when the diff is markdown-only
• keep Playwright workflow behavior unchanged otherwise

Testing

• npm test (unit + demo Playwright)

[SorraTheOrc]

Fixed GITHUB_OUTPUT formatting for changed_files to avoid multiline output parse error in gate job. Change: use heredoc-style block appended to GITHUB_OUTPUT.