Add AGENTS.md with Cursor Cloud development environment instructions

[marcin-kordas-hoc]

Context

Adds AGENTS.md with Cursor Cloud specific instructions for future cloud agents developing on this codebase. Documents Node.js version requirements (v16 via nvm), key commands, testing caveats (public repo has only smoke tests), and how to run the library.

How did you test your changes?

• Installed Node.js v16 via nvm and ran npm install
• Verified npm run lint passes cleanly
• Verified npm run compile (TypeScript compilation) succeeds
• Verified npm run test:jest passes all 4 smoke tests
• Verified npm run bundle:cjs builds CommonJS output
• Ran a hello-world demo exercising PMT, SUM, AVERAGE, IF, CONCATENATE formulas and dependency tracking

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

N/A - Development environment setup

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.