The answer, at least as of TiddlyWiki 5.1.23 with no plugins, is Wrap selection in square brackets.

Here's the filter I used:

[all[shadows]tag[$:/tags/EditorToolbar]get[description]split[{{]split[}}]get[text]!sort[]first[]]

Here's a live version. The {{{triple curly braces}}} tell TiddlyWiki to display the result of the filter; we'll learn much more about this syntax in Filters and Transclusions.

{{{ [all[shadows]tag[$:/tags/EditorToolbar]get[description]split[{{]split[}}]get[text]!sort[]first[]] }}}

You might have used a slightly different filter; as usual with computers, there's more than one way to do things.

Let's break this filter down:

• all[shadows]: As mentioned in the question, you won't be able to see any of the tiddlers we're looking at without adding this. We'll talk more about shadow tiddlers in a later chapter.
• tag[$:/tags/EditorToolbar]: This limits the list of shadow tiddlers to those that represent buttons on the editor toolbar.
• get[description]: This retrieves the contents of the description field of each matching tiddler. Thus, instead of the tiddler titles being in our filter pipeline, the contents of the description fields of each tiddler will now be in the pipeline.
• split[{{]split[}}]: This eliminates the {{double braces}} that surround each value of the description field on these tiddlers. Double braces introduce transclusion, which we'll discuss in a later chapter.
• get[text]: Removing the double braces resulted in a new set of tiddler titles. We want to get the text of these tiddlers, since that contains the descriptions we're looking at.
• !sort[]: This sorts the set of tiddler texts in our filter pipeline in reverse order, putting the alphabetically last item at the top, which is Wrap selection in square brackets.
• first[]: This selects only the first item. You don't really have to use this in this case – you could just pick the first item out of the list – but it'll be nice later when you learn how to display the results of a filter inside some other part of TiddlyWiki's interface and a bunch of irrelevant crud will be much more annoying.