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>

launcher/Makefile

@@ -11,7 +11,7 @@ CFLAGS  = -Wall -Wextra -O2 $(PKG_XFT_C)
 LDFLAGS = $(PKG_XFT_L)
 
 SRC = src/launcher.c
-BIN = skylauncher
+BIN = orionlauncher
 
 .PHONY: all clean install
 all: $(BIN)

launcher/src/launcher.c

@@ -305,7 +305,7 @@ static void draw(L *l) {
     XSetForeground(l->dpy, l->gc, mkc(l, C_ACCENT));
     XFillRectangle(l->dpy, l->win, l->gc, 0, 0, WIN_W, 2);
 
-    ds(l, xd, l->font_logo, C_ACCENT, 14, 13, "SkyOS");
+    ds(l, xd, l->font_logo, C_ACCENT, 14, 13, "OrionOS");
 
     /* search box */
     {
@@ -462,7 +462,7 @@ int main(void) {
     L l = {0}; l.sel_cat = 0; l.hov = -1; l.scroll = 0;
 
     l.dpy = XOpenDisplay(NULL);
-    if (!l.dpy) { fprintf(stderr, "skylauncher: cannot open display\n"); return 1; }
+    if (!l.dpy) { fprintf(stderr, "orionlauncher: cannot open display\n"); return 1; }
     l.screen = DefaultScreen(l.dpy);
     l.root   = RootWindow(l.dpy, l.screen);
     l.cmap   = DefaultColormap(l.dpy, l.screen);