fix: suppress git dirty warnings in direnv skill loading

Add --option warn-dirty false to nix build call to prevent
repeated 'Git tree is dirty' warnings when loading skills.

bin/use-skills.sh

@@ -32,7 +32,7 @@ use_skill() {
   local skill="$1"
   local out
 
-  out=$(nix build --print-out-paths --no-link "${SKILLS_REPO}#${skill}") || {
+  out=$(nix build --option warn-dirty false --print-out-paths --no-link "${SKILLS_REPO}#${skill}") || {
     echo "use_skill: failed to build ${skill}" >&2
     return 1
   }