Migrate linting setup to ESLint v9 (flat config)

[Copilot]

This PR updates the repository’s ESLint stack from v8 to v9 and migrates config to the flat-config model required by modern ESLint. It also removes one rule set that is not currently compatible with ESLint v9.

• ESLint v9 upgrade

  • Bumps eslint to ^9.0.0.
  • Adds @eslint/js and @eslint/eslintrc to support flat config composition.

• Config migration to flat config

  • Replaces package.json eslintConfig with a new root eslint.config.js.
  • Preserves the existing TypeScript/Jest/import rule presets via FlatCompat.

• Lint command compatibility

  • Updates lint script from eslint src --ext .ts to eslint src for v9 behavior.

• Plugin compatibility adjustment

  • Removes eslint-plugin-deprecation and its plugin:deprecation/recommended extend because the published version is ESLint 8–only and crashes under ESLint 9 APIs.

// eslint.config.js (new)
import { FlatCompat } from '@eslint/eslintrc';
import js from '@eslint/js';

const compat = new FlatCompat({ recommendedConfig: js.configs.recommended });

export default [
  ...compat.config({
    parser: '@typescript-eslint/parser',
    parserOptions: { project: './tsconfig.lint.json' },
    extends: [
      'eslint:recommended',
      'plugin:@typescript-eslint/recommended',
      'plugin:jest/recommended',
      'plugin:import/errors',
      'plugin:import/typescript',
    ],
  }),
];