Add workflow to delete merged PR branches (#9)

Author: Zero-1016

.github/workflows/delete-merged-branch.yml

@@ -0,0 +1,23 @@
+name: Delete Merged Branch
+
+on:
+  pull_request:
+    types:
+      - closed
+
+permissions:
+  contents: write
+
+jobs:
+  delete-branch:
+    if: >
+      github.event.pull_request.merged == true &&
+      github.event.pull_request.head.repo.full_name == github.repository &&
+      github.event.pull_request.head.ref != github.event.repository.default_branch &&
+      github.event.pull_request.head.ref != 'main'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Delete merged branch
+        run: |
+          git push "https://x-access-token:${{ secrets.GITHUB_TOKEN }}@github.com/${{ github.repository }}.git" \
+            --delete "${{ github.event.pull_request.head.ref }}" || true