docs: explain automatic generation prune for users (#128)

RECOVERY §4 documents the 14-day / keep-≥3-past policy for system and
Home Manager gens, with dry-run and timer commands. README and
REQUIREMENTS point here so install and day-2 hygiene match the product.

docs/REQUIREMENTS.md

@@ -116,9 +116,13 @@ swapfile, leaving room for store + snapshots.
   is a download/USB cost, not permanent free space on the installed machine
   after install (the installed system has its own store).
 
-**Hygiene (user-facing, later doc links):** delete old generations, run GC,
-tune snapper retention — see [RECOVERY.md](RECOVERY.md) and the System ›
-Recovery menu. This requirements page only states why the disk fills up.
+**Hygiene (automatic on Nomarchy):** a weekly `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),
+then reclaims the store. Details and manual commands:
+[RECOVERY.md §4](RECOVERY.md#4-how-generations-are-kept-and-cleaned-up).
+Also tune snapper retention for BTRFS file history (System › Recovery ›
+Files). This requirements page only states why the disk fills up.
 
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