Add prompt review demo content

[rogerchappel]

Summary

• adds prompt fixtures for a tool-expansion review demo
• adds tutorial, video brief, and social hook drafts grounded in PromptDiff behavior
• links the walkthrough from the README

Verification

• npm test
• node dist/cli.js compare examples/prompts/tool-expansion-old.md examples/prompts/tool-expansion-new.md --out /tmp/promptdiff-tool-expansion.md
• node dist/cli.js compare examples/prompts/tool-expansion-old.md examples/prompts/tool-expansion-new.md --format json >/tmp/promptdiff-tool-expansion.json