Add wallpaper rotation system with upgrade workflow

- Add home/wallpapers/default.nix with per-wallpaper scaling options
- Add 5 new Metroid-themed wallpapers to the rotation
- Update i3+sway and plasma-manager to use wallpaper module
- Add scripts/rotate-wallpaper.sh to cycle through wallpapers
- Add scripts/upgrade.sh to chain: flake update, doom, claude-code, wallpaper
- Add flake apps: rotate-wallpaper, update-claude-code, upgrade
- Fix claude-code update.sh to use REPO_ROOT for flake app compatibility

Run `nix run .#upgrade` for full system update with wallpaper rotation.

home/roles/plasma-manager/default.nix

@@ -4,6 +4,8 @@ with lib;
 
 let
   cfg = config.home.roles.plasma-manager;
+  wallpaperConfig = import ../../wallpapers;
+  currentWallpaper = builtins.elemAt wallpaperConfig.wallpapers wallpaperConfig.currentIndex;
 in
 {
   options.home.roles.plasma-manager = {
@@ -181,7 +183,7 @@ in
         plasma-localerc.Formats.LANG = "en_US.UTF-8";
 
         # Set wallpaper for all desktops
-        plasmarc.Wallpapers.usersWallpapers = "${../../wallpapers/metroid-samus-returns-kz-3440x1440.jpg}";
+        plasmarc.Wallpapers.usersWallpapers = "${currentWallpaper.file}";
       };
     };
   };