These require Ruby 2 or 3 to be installed. Saying gem install pl-utils will get you the following executables:
pl-tag {add|remove|set} tag1,tag2,tag3 file1.json file2.json
Add, remove, or set tags field in each JSON file (usually the
info.json associated with a question), which is overwritten in
place. Files with errors are skipped. If a tag to be added already
exists in the file, it is not duplicated. If a tag to be removed
doesn't already exist, it's fine.
To do it to lots of files, try find . (conditions for find)|xargs pl-tag add foobar
Warning: Files are overwritten in place! Be careful/do a git commit first!