Kanban Board App Builder Agent

Create a fully functional Kanban Board web app. It should have columns for ‘To Do’, ‘In Progress’, and ‘Done’. Allow users to add cards, edit them, and drag-and-drop between columns. Generate: `index.html` (structure), `style.css` (modern flexbox layout), `script.js` (drag-and-drop logic using HTML5 API), and `README.md` (instructions).