Fix lint script setup

[mojobeeping]

Summary

• add ESLint 9 flat-config tooling so the existing npm run lint script can run from a fresh install
• add a minimal ESM/Node flat config for src/ and test/
• remove dead imports and locals surfaced by the recommended lint rules

Fixes #2.

Verification

• rm -rf node_modules
• npm ci --ignore-scripts --no-audit --no-fund
• npm run lint
• npm test
• git diff --check