deps: upgrade dev dependencies to latest

[mancalvo]

Description

Upgrade dev dependencies: chai 5→6, sinon 20→21, ESLint flat config migration.
Remove unused built-in stubs (fs). Add mocha as explicit devDependency.
Replace eslint-plugin-prettier with eslint-config-airbnb-extended and eslint-config-prettier.

Task Context

What is the current behavior?

• ESLint v9 with eslint.config.js and eslint-plugin-prettier
• fs listed as production dependency (Node.js built-in)
• mocha referenced in test script but not installed

What is the new behavior?

• ESLint v9 with eslint.config.mjs and eslint-config-airbnb-extended
• fs removed, mocha added as devDependency
• Standardized Prettier config and pre-commit hooks

Additional Context

No runtime logic changes. Dev tooling and code style only.

[danteay]

LGTM. ESLint flat config migration to eslint-config-airbnb-extended, removal of unused fs built-in stub, mocha added as explicit devDependency, chai 5→6 and sinon 20→21. Auto-fix changes (import order, destructuring shorthand, blank lines). No runtime logic changes. CI passes.