Scaffold Express TODO API with GET /todos and GET /healthz

- Add package.json with express dependency and npm start script
- Add index.js: Express server on port 3000 with /todos and /healthz endpoints
- /todos returns 3 hardcoded todo objects {id, title, done}
- /healthz returns {ok: true}
- Update README.md with Run section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -0,0 +1,12 @@
+{
+  "name": "todo-app",
+  "version": "0.1.0",
+  "description": "Minimal Express TODO API",
+  "main": "index.js",
+  "scripts": {
+    "start": "node index.js"
+  },
+  "dependencies": {
+    "express": "^4.18.2"
+  }
+}