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

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Franz Kafka
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

@@ -31,12 +31,36 @@ upstream_url: "https://api.z.ai/api/anthropic"
 
 ## Building
 
+### Standard Go build
+
 ```bash
 go build -o proxx
 ```
 
 Requires Go 1.21+ with only `gopkg.in/yaml.v3` as an external dependency.
 
+### Nix build (reproducible)
+
+```bash
+nix build
+```
+
+Or run directly from the flake:
+
+```bash
+nix run .
+```
+
+This uses a reproducible build environment with pinned dependencies.
+
+### Nix development shell
+
+```bash
+nix shell
+```
+
+Provides Go toolchain with gopls and other development tools.
+
 ## Running
 
 ```bash

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
+          ];
+        };
+      }
+    );
+}