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>

.gitignore

@@ -1,8 +1,8 @@
 .DS_Store
 *.iso
 *.o
-deskbar/skydeskbar
-installer/skyinstall
-installer/skyinstall-gui
-launcher/skylauncher
-wm/skywm
+deskbar/oriondeskbar
+installer/orioninstall
+installer/orioninstall-gui
+launcher/orionlauncher
+wm/orionwm