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

hosts/framework/hardware-configuration.nix

@@ -0,0 +1,12 @@
+# Replace this entire file with /mnt/etc/nixos/hardware-configuration.nix
+# after running: sudo nixos-generate-config --root /mnt
+# No disks, UUIDs, CPU vendor, or GPU model are guessed here.
+{ ... }:
+{
+  assertions = [
+    {
+      assertion = false;
+      message = "Framework hardware configuration is missing. Run nixos-generate-config --root /mnt and replace hosts/framework/hardware-configuration.nix with the generated file.";
+    }
+  ];
+}