chore(packages): Add linting and code quality automation #441

To ensure code consistency and maintainability, we need to set up automated linting and code quality checks for the JS packages in packages/ (currently toolbar and cli). This setup will serve as the reference standard for future hwptoolkit packages.

Today there's no ESLint or Prettier config in either package or at the repo root, and .github/workflows/code-quality.yml only runs PHP checks for plugins.

Acceptance Criteria

• Set up and configure ESLint, shared across packages/ (not per package)
• Add a shared Prettier config
• Add lint (and format) scripts
• Add a JS job to the Code Quality GitHub Action that runs on changes under packages/

Notes

• Packages ship from source with no build step; ESLint and Prettier are dev/CI only and must not add one.
• Worth landing before the toolbar alpha release (Alpha release of @wpengine/hwp-toolbar NPM package #435).