fix: add eslint tooling for lint script

[xianzuyang9-blip]

Summary

• add ESLint as a dev dependency so the existing npm run lint script has a local binary after a clean install
• add a flat ESLint config for the ESM Node source and tests
• keep the lint baseline conservative by checking parse/undefined errors without forcing existing unused-variable cleanup into this tooling fix

Fixes #2

Verification

• reproduced the clean-install failure before the fix: npm run lint failed with 'eslint' is not recognized as an internal or external command
• npm ci --registry=https://registry.npmjs.org/ --replace-registry-host=never
• npm run lint

Notes

• npm test currently has an existing failure in test/checkers/skill-md.test.js (Expected score >= 5, got 0). I left that out of scope for this lint-tooling fix.