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>

wm/Makefile

@@ -16,7 +16,7 @@ LDFLAGS = $(PKG_XFT_L)
 
 SRC = src/wm.c src/client.c src/events.c src/draw.c src/color.c src/menu.c src/switcher.c
 OBJ = $(SRC:.c=.o)
-BIN = skywm
+BIN = orionwm
 
 .PHONY: all clean install
 all: $(BIN)