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

modules/environments/kde-desktop/default.nix

@@ -11,37 +11,17 @@ in
 {
   options.my.profiles.kde-desktop = with lib; {
     enable = mkEnableOption "KDE Desktop Environment";
-    startOnBoot = mkOption {
-      type = types.bool;
-      default = true;
-      description = ''
-        Whether the display manager (SDDM) starts automatically at boot.
-        When false, the system boots to a text console and KDE can be
-        launched on demand with the `desktop` command.
-      '';
-    };
   };
 
-  config = lib.mkIf cfg.enable (lib.mkMerge [
-    {
-      services = {
-        displayManager.sddm.enable = true;
-        displayManager.sddm.wayland.enable = true;
-        desktopManager.plasma6.enable = true;
-      };
-      users.users.finn.packages = with pkgs; [
-        # Programms can be added here...
-        numix-icon-theme
-      ];
-    }
-    (lib.mkIf (!cfg.startOnBoot) {
-      # Boot to a text console; SDDM stays installed but is not pulled in
-      # by any boot target. Launch KDE on demand with `desktop`.
-      systemd.services.display-manager.wantedBy = lib.mkForce [ ];
-      systemd.defaultUnit = lib.mkForce "multi-user.target";
-      environment.systemPackages = [
-        (pkgs.writeShellScriptBin "desktop" "exec sudo systemctl start display-manager.service")
-      ];
-    })
-  ]);
-}
+  config = lib.mkIf cfg.enable {
+    services = {
+      displayManager.sddm.enable = true;
+      displayManager.sddm.wayland.enable = true;
+      desktopManager.plasma6.enable = true;
+    };
+    users.users.finn.packages = with pkgs; [
+      # Programms can be added here...
+      numix-icon-theme
+    ];
+  };
+}