Add Nix flake for reproducible builds

- Add flake.nix with buildGoModule for reproducible binary builds
- Provide dev shell with Go toolchain (go, gopls, gotools)
- Update README with Nix build and dev shell instructions
- Add MIT LICENSE file

Note: vendorHash is set to null - run `nix build` to generate actual hash

flake.nix

@@ -0,0 +1,42 @@
+{
+  description = "OpenAI-to-Anthropic API proxy";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        packages.default = pkgs.buildGoModule {
+          pname = "proxx";
+          version = "0.1.0";
+          src = ./.;
+
+          # Run: nix-build 2>&1 | grep "got:" to get actual hash
+          # Then replace vendorHash below
+          vendorHash = null;
+
+          meta = with pkgs.lib; {
+            description = "OpenAI-to-Anthropic API proxy";
+            homepage = "https://git.ashisgreat.xyz/penal-colony/proxx";
+            license = licenses.mit;
+            maintainers = [ ];
+          };
+        };
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            go
+            gopls
+            gotools
+            go-tools
+          ];
+        };
+      }
+    );
+}