scripts: add a script to clean the git dir

scripts/clean

@@ -0,0 +1,19 @@
+#!/bin/sh
+# remove artifacts which i've accidentally left lying around
+# e.g. `result -> /nix/store/...` symlinks
+
+pushd ..
+for result in $(fd --follow result); do
+  if [[ "$(readlink "$result")" != /nix/store/* ]]; then
+    # not a build artifact
+    continue
+  fi
+  if [[ "$result" == build/* ]] || [[ "$result" == .working/* ]]; then
+    # intentionally preserved build artifact
+    continue
+  fi
+
+  echo "removing: $result"
+  unlink "$result"
+done
+popd