ci: use merge-mode squash
diff --git a/zuul.d/project.yaml b/zuul.d/project.yaml
index 7037611..50c47f1 100644
--- a/zuul.d/project.yaml
+++ b/zuul.d/project.yaml
@@ -13,6 +13,7 @@
 # under the License.
 
 - project:
+    merge-mode: squash
     check:
       jobs:
         - atmosphere-buildset-registry