update: switch Terraform to NixOS template workflow

- Point clone_template to nixos-template and trim cloud-init to Nix-safe hostname/DNS only
- Remove SSH/Tailscale cloud-init variables and workflow secret dependencies
- Add reusable NixOS template-base config with bootloader, Tailscale, fish, and utility packages

nixos/template-base/flake.nix

@@ -0,0 +1,14 @@
+{
+  description = "Base NixOS config for Proxmox template";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+  };
+
+  outputs = { nixpkgs, ... }: {
+    nixosConfigurations.template = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [ ./configuration.nix ];
+    };
+  };
+}