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/wallpapers/default.nix

@@ -0,0 +1,45 @@
+# Wallpaper rotation system
+# The currentIndex is incremented by `nix run .#rotate-wallpaper`
+# and gets committed as part of `nix run .#upgrade`
+{
+  currentIndex = 1;  # Index into wallpapers list
+
+  wallpapers = [
+    {
+      name = "metroid-samus-returns";
+      file = ./metroid-samus-returns-kz-3440x1440.jpg;
+      sway = "fill";
+      feh = "--bg-fill";
+    }
+    {
+      name = "metroid3_map";
+      file = ./metroid3_map.gif;
+      sway = "fit";
+      feh = "--bg-max";
+    }
+    {
+      name = "super-metroid-gunship-cavern";
+      file = ./super-metroid-gunship-cavern.jpg;
+      sway = "fit";
+      feh = "--bg-max";
+    }
+    {
+      name = "super-metroid-samus-statue";
+      file = ./super-metroid-samus-statue.png;
+      sway = "fit";
+      feh = "--bg-max";
+    }
+    {
+      name = "metroid-samus-action-4k";
+      file = ./metroid-samus-action-4k.jpg;
+      sway = "fit";
+      feh = "--bg-max";
+    }
+    {
+      name = "metroid-creature-minimalist";
+      file = ./metroid-creature-minimalist.jpg;
+      sway = "fit";
+      feh = "--bg-max";
+    }
+  ];
+}