A todo node's key and text were hand-expanded `{field}` strings. They are now Jinja templates rendered through the new hive-template crate (minijinja, an unknown variable is an error), and the controller's save check reads the variables the template uses. The flows document is version 2; a version 1 document reads as version 2 with each `{field}` converted to `{{ field }}`, and the next save writes it.
