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.

pkgs/nomarchy-doctor/nomarchy-doctor.sh

@@ -60,7 +60,7 @@ for mp in / /boot /nix; do
   avail=$(numfmt --to=iec $((availkb * 1024)))
   if [ "$use" -ge 90 ]; then
     bad "$mp is ${use}% full (${avail} free)" \
-        "sudo nix-collect-garbage --delete-older-than 14d  (then sys-rebuild to prune old boot entries)"
+        "sudo nomarchy-gen-prune  (14d + keep ≥3 past system/HM gens; then reclaims store)"
   else
     ok "$mp has space (${use}% used, ${avail} free)"
   fi
@@ -127,6 +127,21 @@ else
   skip "system generation age (no system profile)"
 fi
 
+# ── generation prune (#128: 14d + keep ≥3 past, system + HM) ─────────
+if [ -n "$(systemctl list-unit-files nomarchy-gen-prune.timer --no-legend --plain 2>/dev/null)" ]; then
+  if systemctl is-enabled --quiet nomarchy-gen-prune.timer 2>/dev/null; then
+    ok "generation prune timer is enabled (14d, keep ≥3 past)"
+  else
+    bad "nomarchy-gen-prune.timer is installed but not enabled" \
+        "systemctl enable --now nomarchy-gen-prune.timer"
+  fi
+elif command -v nomarchy-gen-prune >/dev/null 2>&1; then
+  warn "nomarchy-gen-prune is on PATH but no timer unit" \
+       "rebuild the system so gen-prune.nix ships the weekly timer"
+else
+  skip "generation prune (package not on this machine)"
+fi
+
 # ── snapper timeline (when enabled) ──────────────────────────────────
 if [ -n "$(systemctl list-unit-files snapper-timeline.timer --no-legend --plain 2>/dev/null)" ]; then
   if systemctl is-active --quiet snapper-timeline.timer; then