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.

agent/JOURNAL.md

@@ -19,6 +19,17 @@ Template:
 
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+## 2026-07-15 — #128 generation prune (14d + keep ≥3 past, system+HM)
+- **Task:** Implement auto-prune for system and Home Manager generations.
+- **Did:** `pkgs/nomarchy-gen-prune` + `modules/nixos/gen-prune.nix` weekly
+  timer; pure select (current + 3 past always; else drop if age>14d); store
+  GC without `--delete-older-than`; bootloader refresh after system prune.
+  Doctor fix + RECOVERY note. Closed #128.
+- **Verified:** V0/V1 — `checks.gen-prune` (self-test + dry-run + timer
+  assert + no age-only gc options). V3: run on a multi-gen box.
+- **Pending:** V3 on real machine with many gens.
+- **Next suggestion:** human #115/#120, or hardware queue.
+
 ## 2026-07-15 — File #128 generation auto-prune policy (no code)
 - **Task:** Bernardo: auto-clean gens older than N days but always keep
   ≥3 past; include HM; prefer 14d over 30d.