ci: Trigger on merge

Author: halimi

.github/workflows/trigger-os-merge.yml

@@ -1,11 +1,16 @@
 name: Trigger OS merge action in TTI repo
 
-on: pull_request
+on:
+  pull_request:
+    branches:
+      - 'v3.*'
+    types:
+      - closed
 
 jobs:
   trigger_merge:
     name: Trigger OS merge action in TTI repo
-    if: ${{ github.repository == 'TheThingsNetwork/lorawan-stack' }}
+    if: ${{ github.repository == 'TheThingsNetwork/lorawan-stack' && github.event.pull_request.merged == true }}
     runs-on: ubuntu-24.04
     steps:
       - name: Trigger OS merge action in TTI repo