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/picom.conf

@@ -1,6 +1,6 @@
-# SkyOS picom compositor config
+# OrionOS picom compositor config
 # Goal: subtle window shadows, no transparency, fast rendering.
-# BeOS/SkyOS didn't have transparency — we keep it clean.
+# BeOS/OrionOS didn't have transparency — we keep it clean.
 
 backend = "xrender";          # xrender works on all hardware; switch to glx for performance
 vsync = false;