feat(scripts): Add --help flags to all flake apps

Add consistent --help/-h argument handling to update-doomemacs.sh,
rotate-wallpaper.sh, and upgrade.sh scripts. Each script now displays
usage information and a description of what it does.

update-claude-code already had --help support.

scripts/rotate-wallpaper.sh

@@ -1,6 +1,30 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+# Parse arguments
+while [[ $# -gt 0 ]]; do
+    case $1 in
+        --help|-h)
+            echo "Usage: $0 [OPTIONS]"
+            echo ""
+            echo "Rotate to the next wallpaper in the configured list."
+            echo ""
+            echo "This script increments the currentIndex in home/wallpapers/default.nix,"
+            echo "cycling through available wallpapers. Rebuild your system to apply"
+            echo "the new wallpaper."
+            echo ""
+            echo "Options:"
+            echo "  --help, -h    Show this help message"
+            exit 0
+            ;;
+        *)
+            echo "Unknown option: $1"
+            echo "Use --help for usage information"
+            exit 1
+            ;;
+    esac
+done
+
 # Colors for output
 RED='\033[0;31m'
 GREEN='\033[0;32m'