Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,33 @@
+{
+  description = "mcr - Metacircular Container Registry";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+      version = "0.1.0";
+    in
+    {
+      packages.${system} = {
+        default = pkgs.buildGoModule {
+          pname = "mcrctl";
+          inherit version;
+          src = ./.;
+          vendorHash = null;
+          subPackages = [
+            "cmd/mcrctl"
+          ];
+          ldflags = [
+            "-s"
+            "-w"
+            "-X main.version=${version}"
+          ];
+        };
+      };
+    };
+}