add missing sane-nix-files

hmm. not great that i could deploy even without this being checked into git....

pkgs/additional/sane-nix-files/default.nix

@@ -0,0 +1,34 @@
+{
+  findutils,
+  runCommandLocal,
+  rsync,
+}:
+runCommandLocal "sane-nix-files" {
+  nativeBuildInputs = [
+    findutils
+    rsync
+  ];
+
+  meta = {
+    description = "top-level host configs for Colin's machines";
+    longDescription = ''
+      i like to ensure a copy of my config is present on all my machines,
+      and this does that in a hermetic way.
+    '';
+  };
+} ''
+  mkdir src
+  pushd src
+
+  rsync -lptr ${../../../.}/ ./
+  chmod u+w .
+  for pat in $(cat .gitignore); do
+    set +e
+    chmod u+w -R "$pat" ; rm -rf "$pat"
+    find $PWD -name "$pat" -exec 'chmod u+w -R {}; rm -rf {}' \;
+    set -e
+  done
+  rsync -lptr ./ $out/
+
+  popd
+''