Either I am extremely dense or I have some issue with my environment.
I am looking at the files list in the Management panel. If I select and right-click on a file, even one that is part of the project and getting compiled with it, it gives me the option to "Add to Active Project", but not "Remove from Project". It does have a delete option, but I don't want to delete the file completely -- just to remove it from the project.
Also, the Management panel lists all files in the folder, not just those that are explicitly added to the project.