just: parameterize update recipe (system/home/both)

`just update system` re-locks + rebuilds the NixOS system flake,
`just update home` re-locks + switches the Home Manager flake, and
`just update` with no arg does both (system first). Previously update
only touched the system flake, leaving the Home Manager side stale.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/system/justfile.nix

@@ -26,9 +26,29 @@ let
         nix-collect-garbage -d
 
     [no-cd]
-    update:
+    update target="":
+        #!/usr/bin/env bash
+        set -euo pipefail
+
+        update_system() {
           sudo nix flake update --flake "''${NIXOS_FLAKE:-/etc/nixos}"
           just rebuild
+        }
+
+        update_home() {
+          nix flake update --flake "''${XDG_CONFIG_HOME:-$HOME/.config}/home-manager"
+          just switch
+        }
+
+        case "{{target}}" in
+          system) update_system ;;
+          home)   update_home ;;
+          "")     update_system; update_home ;;
+          *)
+            echo "usage: just update [system|home]   (empty = both)" >&2
+            exit 1
+            ;;
+        esac
 
     [no-cd]
     init-home: