Add PromptDiff demo script and launch note

[rogerchappel]

Summary

• add a one-command demo that generates Markdown and JSON reports for the tool-expansion fixture
• link the demo from the README
• add a launch note draft with proof points and limitations

Verification

• npm ci
• npm run build && bash demo/review-tool-expansion.sh && test -s demo/output/tool-expansion-report.md && test -s demo/output/tool-expansion-report.json
• npm test