feat (shells): why3 development shell

shells/why3-dev/flake.nix

@@ -0,0 +1,33 @@
+{
+  description = "Nix flake devshell for why3 development";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
+  };
+
+  outputs =
+    { self, nixpkgs, ... }:
+    let
+      system = "x86_64-linux";
+    in
+    {
+      devShells."${system}".default =
+        let
+          pkgs = import nixpkgs { inherit system; };
+        in
+        pkgs.mkShell {
+          packages = with pkgs; [
+            autoconf
+            ocaml
+            ocamlPackages.findlib
+            ocamlPackages.menhir
+            ocamlPackages.menhirLib
+            ocamlPackages.zarith
+          ];
+
+          shellHook = ''
+            echo "Test.";
+          '';
+        };
+    };
+}