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>

installer/Makefile

@@ -2,10 +2,10 @@ CC      = gcc
 CFLAGS  = -O2 -Wall -Wextra $(shell pkg-config --cflags x11 xft fontconfig)
 LDFLAGS = $(shell pkg-config --libs   x11 xft fontconfig)
 
-all: skyinstall-gui
+all: orioninstall-gui
 
-skyinstall-gui: skyinstall-gui.c
+orioninstall-gui: orioninstall-gui.c
 	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS)
 
 clean:
-	rm -f skyinstall-gui
+	rm -f orioninstall-gui