Add local-only cleanup commands for store stats, chat cleanup, group pruning, and age-based store cleanup. Rework group pruning so targets are listed first, dry-run never deletes, confirmation gates every destructive path, and active stale groups require an explicit include flag.
Document the cleanup workflow across README and docs/, including the local-only semantics.
Closes#210.
Co-authored-by: Davy <95214375+thedavidweng@users.noreply.github.com>
