-noupgrade prefix for files that will not be replaced when upgrading code

# Use case

In clonned zites, there is a "Upgrade code" button, where user can receive the new js/css files from the clone origin.
In FrameworkJS, there will be custom CSS/JS, that user can customize. But when upgrading code, those files are also upgraded.

Start at the Upgrade code flow for cloned zites and inspect how FrameworkJS custom CSS and JS files are selected for replacement. Done means a documented -noupgrade prefix or equivalent rule prevents marked files from being upgraded while other files still receive updates.

Vom Indexierungsmodell aus dem Issue-Text verfasst.