feat(mibook): boot to terminal, launch KDE on demand

Add my.profiles.kde-desktop.startOnBoot (default true, no change for
existing consumers). When false, boot to multi-user.target, keep SDDM
out of any boot target's wants, and provide a 'desktop' command that
starts the display manager on demand. Enable this on mibook.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CfozKLQdUh5TzqyjSigLUx

machines/mibook/environments.nix

@@ -6,6 +6,7 @@ in
 {
   my.profiles = {
     kde-desktop.enable = true;
+    kde-desktop.startOnBoot = false;
     zsh.enable = true;
     apps = {
       desktop_apps = true;