Side projects have a specific failure mode: nobody tells you the thing you shipped at midnight has a bug, or that the "quick fix" actually broke three other things. There's no teammate around to catch it.
Pair a Feature Engineer workspace with a Reviewer workspace using the Engineer/Reviewer Strategy, and you get exactly that second set of eyes — one agent builds and hands off, and a separate one checks the work against what you actually asked for before anything merges.
It's not a replacement for a real collaborator. But it catches the kind of thing a tired solo developer waves through at 1am.