feat: implement nixos-exporter

Prometheus exporter for NixOS-specific metrics including:
- Generation collector: count, current, booted, age, config mismatch
- Flake collector: input age, input info, revision behind

Includes NixOS module, flake packaging, and documentation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,53 @@
+{
+  description = "Prometheus exporter for NixOS-specific metrics";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" ];
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+    in
+    {
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.buildGoModule {
+            pname = "nixos-exporter";
+            version = "0.1.0";
+            src = ./.;
+            vendorHash = "sha256-NnvB20rORPS5QF5enbb5KpWaKZ70ybSgfd7wjk21/Cg=";
+
+            meta = with pkgs.lib; {
+              description = "Prometheus exporter for NixOS-specific metrics";
+              homepage = "https://git.t-juice.club/torjus/nixos-exporter";
+              license = licenses.mit;
+              maintainers = [ ];
+              platforms = platforms.linux;
+            };
+          };
+        });
+
+      devShells = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            buildInputs = with pkgs; [
+              go
+              gopls
+              golangci-lint
+              govulncheck
+              delve
+            ];
+          };
+        });
+
+      nixosModules.default = import ./module.nix { inherit self; };
+    };
+}