Phase 7: rename SkyOS → OrionOS across entire codebase

- Rename all binaries: skywm→orionwm, skydeskbar→oriondeskbar,
  skylauncher→orionlauncher, skyinstall/skyinstall-gui→orioninstall*
- Rename theme directory: themes/SkyOS/ → themes/OrionOS/
- Update all string literals, window titles, menu labels, and comments
- Update ISO build: hostname→orionos, user→orion, all paths/labels
- Add Plymouth boot splash theme (orionos) with Navy+Blue palette
- Update GTK2/GTK3 theme names, wallpaper output name
- Update .gitignore, root Makefile, build-iso.sh for new binary names

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

themes/wallpapers/generate-default.sh

@@ -1,9 +1,9 @@
 #!/usr/bin/env bash
-# Generates the default SkyOS wallpaper as a PNG using ImageMagick.
+# Generates the default OrionOS wallpaper as a PNG using ImageMagick.
 # On the live system: sudo pacman -S imagemagick
-# Output: skyos-default.png (1920x1080 — scales to any resolution via feh)
+# Output: orionos-default.png (1920x1080 — scales to any resolution via feh)
 
-OUT="$(dirname "$0")/skyos-default.png"
+OUT="$(dirname "$0")/orionos-default.png"
 
 if ! command -v convert &>/dev/null; then
     echo "ImageMagick not found. Install it, or place any PNG at:"