Add flake.nix for reproducible NixOS builds (engine + converter python env) (#66)

Adds a Nix flake pinned to nixos-26.05. Builds the glm engine with
gcc+OpenMP (AVX2 portable), wraps the coli Python CLI with pinned deps
(torch, safetensors, huggingface-hub, numpy), and provides a dev shell.

flake.nix

@@ -0,0 +1,119 @@
+{
+  description = "colibrì — run GLM-5.2 (744B MoE) on a consumer machine with ~25 GB RAM";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+
+        # Python with the packages needed by the offline converter tools
+        pythonEnv = pkgs.python3.withPackages (ps: with ps; [
+          torch
+          safetensors
+          huggingface-hub
+          numpy
+        ]);
+
+        colibri = pkgs.stdenv.mkDerivation {
+          pname = "colibri";
+          version = "1.0";
+          src = ./.;
+
+          nativeBuildInputs = [ pkgs.makeWrapper ];
+
+          buildInputs = [
+            pkgs.gcc
+            pkgs.gmp
+          ];
+
+          # Use x86-64-v3 (AVX2) for a portable binary; override with ARCH=native for local builds
+          ARCH = "x86-64-v3";
+
+          buildPhase = ''
+            runHook preBuild
+            make -C c glm ARCH="$ARCH"
+            runHook postBuild
+          '';
+
+          installPhase = ''
+            runHook preInstall
+            mkdir -p $out/bin
+            cp c/glm $out/bin/glm
+
+            # Wrap coli (the Python CLI) so it finds the right python and the engine
+            mkdir -p $out/share/colibri
+            cp c/coli $out/share/colibri/coli
+            chmod +x $out/share/colibri/coli
+            cp -r c/tools $out/share/colibri/tools
+
+            makeWrapper ${pythonEnv}/bin/python $out/bin/coli \
+              --add-flags "$out/share/colibri/coli" \
+              --set PYTHONPATH "${pythonEnv}/${pkgs.python3.sitePackages}"
+            runHook postInstall
+          '';
+
+          checkPhase = ''
+            runHook preCheck
+            cd c
+            make test-c
+            cd ..
+            runHook postCheck
+          '';
+
+          doCheck = true;
+
+          meta = with pkgs.lib; {
+            description = "Run GLM-5.2 (744B MoE) on a consumer machine with ~25 GB RAM";
+            homepage = "https://github.com/JustVugg/colibri";
+            license = licenses.asl20;
+            platforms = platforms.linux;
+            mainProgram = "glm";
+          };
+        };
+      in
+      rec {
+        packages = {
+          default = colibri;
+          inherit colibri;
+        };
+
+        apps = {
+          default = {
+            type = "app";
+            program = "${colibri}/bin/glm";
+          };
+          coli = {
+            type = "app";
+            program = "${colibri}/bin/coli";
+          };
+        };
+
+        devShells.default = pkgs.mkShell {
+          inputsFrom = [ colibri ];
+
+          packages = [
+            pythonEnv
+            pkgs.gcc
+            pkgs.gnumake
+            pkgs.clang-tools          # clangd / clang-tidy for IDE support
+            pkgs.pkg-config
+          ];
+
+          shellHook = ''
+            echo "🐦 colibrì dev shell"
+            echo "  gcc: $(gcc --version | head -1)"
+            echo "  python: $(python3 --version)"
+            echo ""
+            echo "Build the engine:   make -C c glm"
+            echo "Run the converter:  python c/coli convert --model /path/to/glm52_i4"
+            echo "Chat:               COLI_MODEL=/path/to/glm52_i4 ./c/glm ..."
+          '';
+        };
+      }
+    );
+}