chore: add ESLint tooling with TypeScript support

The repository has a pnpm lint script running 'eslint .' but ESLint
and typescript-eslint were not listed in devDependencies.

Added:
- eslint (^9.0.0)
- @eslint/js (^9.0.0)
- typescript-eslint (^8.0.0)

Added eslint.config.mjs with flat config (ESLint 9+) using
typescript-eslint strict rules. Ignores node_modules, dist, build,
and framework-specific output directories.

eslint.config.mjs

@@ -0,0 +1,29 @@
+import eslint from '@eslint/js';
+import tseslint from 'typescript-eslint';
+
+export default tseslint.config(
+  eslint.configs.recommended,
+  ...tseslint.configs.strict,
+  {
+    files: ['**/*.ts', '**/*.tsx', '**/*.mts', '**/*.cts'],
+    languageOptions: {
+      parserOptions: {
+        projectService: {
+          allowDefaultProject: ['*.config.*'],
+        },
+      },
+    },
+  },
+  {
+    ignores: [
+      '**/node_modules/**',
+      '**/dist/**',
+      '**/build/**',
+      '**/.understand-anything/**',
+      '**/.claude-plugin/**',
+      '**/.cursor-plugin/**',
+      '**/.copilot-plugin/**',
+      '**/coverage/**',
+    ],
+  },
+);

package.json

@@ -12,7 +12,10 @@
     "lint": "eslint ."
   },
   "devDependencies": {
+    "@eslint/js": "^9.0.0",
+    "eslint": "^9.0.0",
     "typescript": "^5.7.0",
+    "typescript-eslint": "^8.0.0",
     "vitest": "^3.1.0"
   },
   "pnpm": {