fix: restore local eslint config for standalone checkouts

[lawyered0]

Why this change

npm run lint fails in a clean clone of this repository because eslint.config.js imports ../_config/eslint.base.js, but that path is not present in this project.

That means contributors (and CI setups that lint from this repo alone) cannot run lint successfully.

Root cause

eslint.config.js was refactored to depend on a shared config path used in another local setup, but that dependency is not checked into this repository.

What this PR changes

• removes the broken external import from eslint.config.js
• restores a self-contained ESLint flat config in this repo
• keeps the same local rule intent for:
  • src/**/*.js (Node/CommonJS globals + project rules)
  • tests/**/*.js (test/module globals + test rules)
  • ignore paths (node_modules/, coverage/, dist/)

Why this is safe

• scope is limited to one tooling file: eslint.config.js
• no runtime source files are changed
• behavior restored to the previously working local config approach (equivalent to the prior standalone version)

Reproduction (before)

1. npm install
2. npm run lint
3. ESLint throws: Cannot find module '../_config/eslint.base.js'

Verification (after)

• npm run lint completes successfully (2 existing non-blocking warnings in src/patterns.js)
• npm test passes (128/128)
• npm run format:check passes