Add Nix development shell

shell.nix

@@ -0,0 +1,14 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+pkgs.mkShell {
+  packages = with pkgs; [
+    go
+    gopls
+  ];
+
+  shellHook = ''
+    echo "DeathWatch development shell"
+    echo "Go: $(go version)"
+    echo "Run: go test ./..."
+  '';
+}