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/ROADMAP.md

@@ -427,6 +427,11 @@ Design/decision records and a running log of shipped work (items marked
   default `$TERMINAL` / SUPER+Return, doctor + calendar + what-changed use
   `kitty --class=com.nomarchy.*`. Ghostty module and install-time GL probe
   removed. V3: Acer SUPER+Return + doctor float.
+- ✓ **Generation prune (#128):** weekly `nomarchy-gen-prune` (system + HM):
+  delete only gens older than **14 days** that are also beyond the **3 most
+  recent past** gens (current always kept). Stock `nix.gc` no longer uses
+  `--delete-older-than` (floor-free). `checks.gen-prune` self-test + unit
+  wiring. Manual: `sudo nomarchy-gen-prune` / `--dry-run`.
 - ✓ **Recovery + System IA + Control Center retired (#111 / #105 / #110):**
   System menu is Connectivity / Devices / Recovery / Preferences (root still
   six entries). Recovery is scope-first: Desktop generation (HM), System boot