revert(mibook): remove boot-time terminal/desktop choice feature

Back out the entire "choose terminal-only vs KDE at boot" work. The first
attempt (default multi-user.target + on-demand desktop) locked mibook out —
it is WiFi-only with credentials in KWallet, so a headless boot has no
network and no SSH. The follow-up specialisation redesign is no longer
wanted either.

mibook returns to plain always-KDE boot (the pre-feature baseline):
- machines/mibook/configuration.nix: drop specialisation.terminal and the
  NetworkManager-wait-online override.
- machines/mibook/environments.nix: drop kde-desktop.startOnBoot.
- modules/environments/kde-desktop/default.nix: drop the startOnBoot option.
- remove the boot-desktop-choice design spec.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1wvPy5kbGp8pUQX61WBCY

machines/mibook/environments.nix

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