chore: add ESLint configuration to catch code quality issues

Adds comprehensive ESLint setup to proactively catch common code quality
issues flagged by GitHub Copilot during PR reviews, including:

- Implicit any types and unsafe type operations
- Floating promises and misused promises
- Unused variables
- Jest best practices

The configuration is tuned for this TypeScript monorepo and includes
lenient rules for test files where stricter typing is less critical.

This change helps prevent issues like those caught in twilio-labs#546 where Copilot
identified type safety and promise handling concerns.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: deshartman

.eslintrc.js

@@ -0,0 +1,93 @@
+module.exports = {
+  root: true,
+  env: {
+    node: true,
+    es6: true,
+  },
+  plugins: ['@typescript-eslint', 'jest'],
+  extends: ['eslint:recommended', 'plugin:jest/recommended'],
+  rules: {
+    // Best practices
+    'no-console': 'off', // CLI tool, console is expected
+    'prefer-const': 'error',
+    'no-var': 'error',
+
+    // Jest best practices
+    'jest/expect-expect': 'warn',
+    'jest/no-disabled-tests': 'warn',
+    'jest/no-focused-tests': 'error',
+  },
+  overrides: [
+    {
+      // TypeScript files with type checking
+      files: ['**/*.ts', '**/*.tsx'],
+      parser: '@typescript-eslint/parser',
+      parserOptions: {
+        ecmaVersion: 2018,
+        sourceType: 'module',
+        project: ['./tsconfig.base.json', './packages/*/tsconfig.json'],
+        tsconfigRootDir: __dirname,
+      },
+      extends: [
+        'eslint:recommended',
+        'plugin:@typescript-eslint/recommended',
+        'plugin:@typescript-eslint/recommended-requiring-type-checking',
+        'plugin:jest/recommended',
+      ],
+      rules: {
+        // Catch implicit any (would catch Issue #1)
+        '@typescript-eslint/no-explicit-any': 'warn', // Warn, not error (any is sometimes needed)
+        '@typescript-eslint/no-unsafe-assignment': 'warn',
+        '@typescript-eslint/no-unsafe-member-access': 'warn',
+        '@typescript-eslint/no-unsafe-call': 'warn',
+
+        // Catch promise issues
+        '@typescript-eslint/no-floating-promises': 'error',
+        '@typescript-eslint/no-misused-promises': 'error',
+
+        // General code quality
+        '@typescript-eslint/no-unused-vars': [
+          'error',
+          {
+            argsIgnorePattern: '^_',
+            varsIgnorePattern: '^_',
+          },
+        ],
+
+        // Best practices
+        'no-console': 'off',
+        'prefer-const': 'error',
+        'no-var': 'error',
+
+        // Jest best practices
+        'jest/expect-expect': 'warn',
+        'jest/no-disabled-tests': 'warn',
+        'jest/no-focused-tests': 'error',
+      },
+    },
+    {
+      // More lenient rules for test files
+      files: ['**/__tests__/**', '**/*.test.ts', '**/*.test.js'],
+      env: {
+        jest: true,
+      },
+      rules: {
+        '@typescript-eslint/no-explicit-any': 'off',
+        '@typescript-eslint/no-unsafe-assignment': 'off',
+        '@typescript-eslint/no-unsafe-member-access': 'off',
+        '@typescript-eslint/no-unsafe-call': 'off',
+      },
+    },
+  ],
+  ignorePatterns: [
+    'node_modules/',
+    'dist/',
+    'build/',
+    'coverage/',
+    '*.config.js',
+    '.eslintrc.js',
+    'jest.config.js',
+    'jest.config.base.js',
+    '.changeset/',
+  ],
+};

package.json

@@ -19,7 +19,9 @@
     "docs": "npm run docs --workspaces --if-present",
     "prepare": "husky install",
     "pre-commit": "lint-staged",
-    "reset": "npm exec --workspaces -- npx rimraf node_modules && npx rimraf node_modules"
+    "reset": "npm exec --workspaces -- npx rimraf node_modules && npx rimraf node_modules",
+    "lint": "eslint . --ext .ts,.js,.tsx,.jsx",
+    "lint:fix": "eslint . --ext .ts,.js,.tsx,.jsx --fix"
   },
   "devDependencies": {
     "@changesets/changelog-github": "^0.4.8",
@@ -28,11 +30,15 @@
     "@commitlint/config-conventional": "^19.1.0",
     "@twilio/test-dep": "npm:twilio@4.22.0",
     "@types/jest": "^29.2.4",
+    "@typescript-eslint/eslint-plugin": "^6.21.0",
+    "@typescript-eslint/parser": "^6.21.0",
     "all-contributors-cli": "^6.1.2",
     "commitizen": "^4.2.4",
     "commitlint-plugin-workspace-scopes": "^1.1.0",
     "conventional-changelog-cli": "^2.1.0",
     "cz-conventional-changelog": "^2.1.0",
+    "eslint": "^8.57.1",
+    "eslint-plugin-jest": "^27.9.0",
     "husky": "^8.0.2",
     "jest": "^29.7.0",
     "jest-express": "^1.10.1",
@@ -47,6 +53,7 @@
   },
   "lint-staged": {
     "*.{js,jsx,ts,tsx}": [
+      "eslint --fix",
       "prettier --write",
       "git add"
     ]