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

flake.nix

@@ -0,0 +1,45 @@
+{
+  description = "Chris's NixOS machines";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
+    # Only the existing hosts use this for apps previously taken from unstable.
+    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
+    nixos-hardware = {
+      url = "github:NixOS/nixos-hardware";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      nixpkgs-unstable,
+      nixos-hardware,
+      ...
+    }:
+    {
+      nixosConfigurations = {
+        t450s = nixpkgs.lib.nixosSystem {
+          specialArgs.unstable = import nixpkgs-unstable {
+            system = "x86_64-linux";
+            config.allowUnfree = true;
+          };
+          modules = [ ./hosts/t450s/configuration.nix ];
+        };
+        x1eg2 = nixpkgs.lib.nixosSystem {
+          specialArgs.unstable = import nixpkgs-unstable {
+            system = "x86_64-linux";
+            config.allowUnfree = true;
+          };
+          modules = [ ./hosts/x1eg2/configuration.nix ];
+        };
+        framework = nixpkgs.lib.nixosSystem {
+          modules = [
+            nixos-hardware.nixosModules.framework-11th-gen-intel
+            ./hosts/framework/configuration.nix
+          ];
+        };
+      };
+    };
+}