chore(ci): rework mergify configuration

Author: selfuryon

.github/mergify.yml

@@ -1,12 +1,15 @@
-defaults:
-  actions:
-    queue:
-      allow_merging_configuration_change: true
-
 queue_rules:
   - name: default
     conditions:
-      - check-success=buildbot/nix-eval
+      - "check-success=buildbot/nix-eval"
+    batch_size: 1
+    merge:
+      method: squash
+      commit_message_template: |
+        {{ title }} (#{{ number }})
+        
+        {{ body }}
+      allow_merging_configuration_change: true
 
 pull_request_rules:
   - name: Automerge PRs using the merge queue
@@ -20,6 +23,7 @@ pull_request_rules:
 
   - name: Delete head branch after merge
     conditions:
-      - "merged"
+      - merged
     actions:
-      delete_head_branch: {}
+      delete_head_branch:
+        force: true