As long as this is your 'root/todos/' directory, use this:
<!DOCTYPE html>
<html>
<head></head>
<body>
<ul id='todos'></ul>
<input id='editor'/>
<script src='../steal/steal.js?todos/todos.js'></script>
</body>
</html>
I think you have an extra dot inside:
<script src='.../steal/steal.js?todos/todos.js'></script>