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

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