feat(nixos): consolidate hosts into a shared flake

Manage T450s, X1EG2 and Framework 13 from pinned NixOS inputs while retaining
machine-specific storage and hardware configuration.

Changes:

Shared Configuration:
- Extract common, KDE, desktop, development and Steam modules
- Share Emacs, VS Code, htop, DevOps tools and Docker across hosts
- Remove obsolete apps and global project language runtimes

Framework:
- Import the 11th-generation Intel hardware profile
- Enable Steam and ZFS with tookland/home mounted at /home
- Document manual ext4 root and ZFS installation steps
- Require installer-generated hardware configuration before installation

Validation:
- Evaluate both ThinkPads and Framework with test hardware
- Check Nix syntax, formatting and whitespace

modules/kde.nix

@@ -0,0 +1,23 @@
+{ ... }:
+{
+  services.xserver.enable = true;
+  services.xserver.xkb = {
+    layout = "us";
+    variant = "";
+  };
+  services.displayManager.sddm.enable = true;
+  services.desktopManager.plasma6.enable = true;
+  programs.dconf.enable = true;
+  services.printing.enable = true;
+  services.flatpak.enable = true;
+  hardware.bluetooth.enable = true;
+
+  security.rtkit.enable = true;
+  services.pipewire = {
+    enable = true;
+    alsa.enable = true;
+    alsa.support32Bit = true;
+    pulse.enable = true;
+    jack.enable = true;
+  };
+}