Add Data Folder

Adds shared data that belongs to no page: a folder document type editors create at the root of the content tree (authorFolder) and the item type it holds (author). Pages then pick items from it instead of retyping the same name on every page. It works requirements-first: write the change down, stop for the user's approval, apply it, then read it back.

"Folder" here is a document type that holds content nodes, not the organisational folders in the Settings tree. The schema of the folder and item types is decided by umbraco-add-document-type with the role data folder or item; this skill adds the content-tree behaviour and the picker.

The rules come from umbraco-content-model-conventions, and the requirements doc format and the six-step workflow from umbraco-content-requirements-documentation (change-workflow.md); both must be installed alongside this one. If a link into either cannot be read, stop and say so.