fix: skip frontend tests when frontend dir absent in sparse worktrees

script/test unconditionally cd'd into frontend/ and ran npm test, which
failed in sparse-checkout worktrees that only contain the server/ subtree.
Guard the frontend step with a directory existence check so acceptance
gates pass for worktrees that have no frontend checkout.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

script/test

@@ -8,8 +8,12 @@ echo "=== Running Rust tests ==="
 cargo test --manifest-path "$PROJECT_ROOT/Cargo.toml"
 
 echo "=== Running frontend unit tests ==="
+if [ -d "$PROJECT_ROOT/frontend" ]; then
   cd "$PROJECT_ROOT/frontend"
   npm test
+else
+  echo "Skipping frontend tests (no frontend directory)"
+fi
 
 # Disabled: e2e tests may be causing merge pipeline hangs (no running server
 # in merge workspace → Playwright blocks indefinitely). Re-enable once confirmed.