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.

README.md

@@ -380,6 +380,13 @@ fonts. No runtime patching means no partial states, and `home-manager
 generations` is also your theme history. Waybar even restyles in place: it
 re-reads `style.css` when the symlink flips.
 
+**Cleanup is automatic.** 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 — so you always keep the
+current generation plus at least three rollbacks, even on a rarely rebuilt
+machine. Manual: `sudo nomarchy-gen-prune --dry-run`. Full story:
+[docs/RECOVERY.md §4](docs/RECOVERY.md#4-how-generations-are-kept-and-cleaned-up).
+
 The **wallpaper** is the one runtime piece (awww — nixpkgs' swww — is
 imperative; nothing in Nix consumes the path): applied at session start,
 after every switch via a tiny activation hook, and again when a monitor is