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/i3+sway/default.nix

@@ -4,6 +4,8 @@ with lib;
 
 let
   cfg = config.home.roles.i3_sway;
+  wallpaperConfig = import ../../wallpapers;
+  currentWallpaper = builtins.elemAt wallpaperConfig.wallpapers wallpaperConfig.currentIndex;
 
   shared_config = recursiveUpdate rec {
     modifier = "Mod4";
@@ -281,7 +283,7 @@ in {
           }
           # Set wallpaper with feh
           {
-            command = "feh --bg-scale ${../../wallpapers/metroid-samus-returns-kz-3440x1440.jpg}";
+            command = "feh ${currentWallpaper.feh} ${currentWallpaper.file}";
             always = false;
             notification = false;
           }
@@ -314,7 +316,7 @@ in {
         };
         output = {
           "*" = {
-            bg = "${../../wallpapers/metroid-samus-returns-kz-3440x1440.jpg} fill";
+            bg = "${currentWallpaper.file} ${currentWallpaper.sway}";
           };
         };
         startup = [