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/gaming.nix

@@ -0,0 +1,9 @@
+{ ... }:
+{
+  # Import on machines where Steam is wanted.
+  programs.steam = {
+    enable = true;
+    remotePlay.openFirewall = true;
+    dedicatedServer.openFirewall = true;
+  };
+}