build: add TypeScript + ESLint baseline (typecheck/lint/test scripts)

Root package.json (type: module), strict tsconfig with allowJs/checkJs:false for
gradual migration, flat ESLint config scoped to scripts/ + tests/. App untouched.
All three green: lint, typecheck, test 107/107.

eslint.config.js

@@ -0,0 +1,35 @@
+// Flat ESLint config for the Obelisk root (Core + skill runtime + tests).
+// Scope: the ESM sources under scripts/ and tests/. The Electron app has its own
+// package and toolchain and is intentionally excluded (see docs/adr/0003).
+// typescript-eslint is deferred to Phase 4, when the first .ts files land.
+
+import js from '@eslint/js';
+import globals from 'globals';
+
+export default [
+  {
+    ignores: [
+      'node_modules/**',
+      'app/**',
+      'dist/**',
+      'release/**',
+      '.dev.docs/**',
+      '.obelisk/**',
+      '.claude/**',
+    ],
+  },
+  js.configs.recommended,
+  {
+    files: ['**/*.{js,mjs}'],
+    languageOptions: {
+      ecmaVersion: 2023,
+      sourceType: 'module',
+      globals: { ...globals.node },
+    },
+    rules: {
+      // Empty catch is an intentional pattern here (best-effort JSON.parse etc.).
+      'no-empty': ['error', { allowEmptyCatch: true }],
+      'no-unused-vars': ['warn', { argsIgnorePattern: '^_', varsIgnorePattern: '^_' }],
+    },
+  },
+];