Add sops-nix for secrets management

- Add flake.nix with sops-nix input
- Configure sops with age key encryption
- Add .sops.yaml template for age key configuration
- Create secrets/ directory for encrypted secrets
- Add .gitignore for age keys and nix result symlinks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,19 @@
+{
+  description = "NixOS VPS configuration";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+    sops-nix.url = "github:Mic92/sops-nix";
+    sops-nix.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, sops-nix, ... }@inputs: {
+    nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        ./configuration.nix
+        sops-nix.nixosModules.sops
+      ];
+    };
+  };
+}