Add AGENTS.md for Cursor Cloud development environment setup

[evermake]

Summary

Adds AGENTS.md with Cursor Cloud specific instructions for developing in this codebase.

What's included

• Key commands reference (lint, typecheck, test, build)
• Non-obvious gotchas:
  • typescript is only a transitive dependency (not directly in devDependencies), so tsc needs global install
  • esbuild postinstall warning from pnpm is cosmetic
  • Vitest runs in watch mode by default
  • Package is ESM-only

Environment verification

All checks pass:

• pnpm run lint — clean
• pnpm run typecheck — clean
• pnpm run test -- --run — 55/55 tests pass
• pnpm run build — produces dist/ output
• Hello-world JSX-to-HTML rendering verified with built output