[tomberek]

Supporting rollback and history means disks can fill up. With Nix, this is usually due to a bunch of GC roots pointing to profiles and packages that then cannot be cleaned up. Our environments are not just symlinks, but have a declarative format (under the hood, flakes) so one can remove them, but also reproduce them as needed. So GC'ing them is less destructive than with using `nix-env`/`nix profile`. This allows us to be more aggressive in cleaning up old generations.

So the strategy is to ensure there is always a declarative+reproducible way to recover the things you clean up, then we can apply various heuristics to avoid disks from filling up; free space, age, least-recently-used, least-frequently-used - are all being considered.