Search by collaborator

Email addresses of collaborators are managed in the same way as regular tags, yet they do not appear in the Sidebar with rest of tags (understandably, but then a special folder there, gathering the list of all collaborators would be nice) and performing a search with their email address returns no note, whereas I expected to get a list of all the notes I am collaborating with that person on.

At the very least, please add a search by collaborator feature.

Start by locating the macOS search handling and the collaborator email/tag data described in the issue, then inspect how notes are filtered and how tags appear in the Sidebar. Clarify whether the completed feature should support email searches, a collaborator folder, or both; done means collaborator searches return all matching shared notes with tests covering the behavior.

Written by the indexing model from the issue text.