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.

packages/claude-code/update.sh

@@ -33,7 +33,8 @@ YELLOW='\033[1;33m'
 NC='\033[0m' # No Color
 
 CASK_URL="https://raw.githubusercontent.com/Homebrew/homebrew-cask/HEAD/Casks/c/claude-code.rb"
-NIX_FILE="$(dirname "$0")/default.nix"
+REPO_ROOT="${REPO_ROOT:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+NIX_FILE="$REPO_ROOT/packages/claude-code/default.nix"
 
 echo "Fetching latest claude-code version from Homebrew cask..."