Add batch boundary demo and social hooks

[rogerchappel]

Summary

• adds a README demo recipe for single-path and JSONL batch checks
• adds docs/tutorials/batch-boundary-demo.md using the fixture root and batch input
• adds docs/promo/social-hooks.md with factual copy and limitations

Verification

• npm install
• npm run build && node dist/src/cli.js check test/fixtures/root/allowed/file.txt --root test/fixtures/root --allow 'allowed/' --deny 'blocked/' && node dist/src/cli.js check test/fixtures/root/blocked/secret.txt --root test/fixtures/root --allow 'allowed/' --deny 'blocked/' --json; code=$?; test $code -eq 1 && node dist/src/cli.js batch --root test/fixtures/root --input examples/batch.jsonl --allow 'allowed/' --deny 'blocked/' --json > /tmp/pathsafe-batch.json; code=$?; test $code -eq 1 && test -s /tmp/pathsafe-batch.json
• npm test

Notes

• Denied path checks and the mixed batch fixture exit non-zero by design.