Add auto-delete branch workflow

Closes: #109

Author: donbarbos

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

@@ -0,0 +1,24 @@
+name: Delete "update" branch after merge
+
+on:
+  pull_request:
+    types: [closed]
+
+jobs:
+  delete-branch:
+    if: >
+      github.event.pull_request.merged == true &&
+      startsWith(github.event.pull_request.head.ref, 'updates/update_')
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Delete branch
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          BRANCH="${{ github.event.pull_request.head.ref }}"
+          echo "Deleting branch: $BRANCH"
+          gh api \
+            -X DELETE \
+            repos/${{ github.repository }}/git/refs/heads/$BRANCH