docs: fix Cursor skill install path

[CooperSheroy]

Summary

• update the manual Cursor install path to use a skill directory instead of a .cursor/rules file
• keep SKILL.md and references/ together so the referenced Hallmark rules remain available to the assistant

Closes #22.

Validation

• git diff --check
• node -e "JSON.parse(require('fs').readFileSync('package.json','utf8')); console.log('package.json valid')"
• rg -n "Cursor|\\.cursor/rules|\\.agents/skills" README.md

[vercel]

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