feat(nixos): prune system+HM gens after 14d, keep ≥3 past (#128)

Weekly nomarchy-gen-prune deletes only generations that are both older
than 14 days and beyond the three most recent past gens (current always
kept), for system and Home Manager profiles. Stock nix.gc no longer uses
--delete-older-than. checks.gen-prune covers selection self-test and
unit wiring; doctor points at the new tool when disk is tight.

docs/RECOVERY.md

@@ -65,6 +65,11 @@ broken one. Make the fix stick from the working boot: revert the change
 in `~/.nomarchy` (`git -C ~/.nomarchy revert …` or edit `system.nix`
 back), then `nomarchy-rebuild`.
 
+**Automatic prune (#128):** a weekly timer (`nomarchy-gen-prune`) removes
+system *and* Home Manager generations that are **older than 14 days** and
+**beyond the three most recent past gens** (current + ≥3 past always kept).
+Manual: `sudo nomarchy-gen-prune` (or `--dry-run`).
+
 ## 4. Files went missing or wrong — snapshots (BTRFS installs)
 
 With `nomarchy.system.snapper.enable` (the installer's default on