Add PromptDiff demo and launch note

[rogerchappel]

Summary

• add a fixture-backed tool expansion demo script that writes Markdown, JSON, and rules reports
• document the repeatable demo in the README
• add a grounded launch note draft for promotion

Verification

• npm ci
• bash demo/run-tool-expansion-review.sh
  • Markdown report: /Users/roger/.openclaw/tmp/promptdiff-tool-expansion/tool-expansion-review.md
  • JSON report: /Users/roger/.openclaw/tmp/promptdiff-tool-expansion/tool-expansion-review.json
  • Rules report: /Users/roger/.openclaw/tmp/promptdiff-tool-expansion/safe-prompt-rules.md