ci: add on-close cleanup for merged bot branches

Deletes the head branch when a merged PR was authored by pre-commit-ci[bot]
or dependabot[bot]. Runs after the merge completes, so deletion is reliable
(the repo-level delete_branch_on_merge setting did not fire for these bot
auto-merges). Mirrors tablackburn/SrrDBAutomationToolkit#27.

Author: tablackburn

.github/workflows/delete-merged-bot-branches.yml

@@ -0,0 +1,22 @@
+name: Delete merged bot branches
+
+on:
+  pull_request:
+    types: [closed]
+
+permissions:
+  contents: write
+
+jobs:
+  delete-branch:
+    if: >
+      github.event.pull_request.merged == true &&
+      (github.event.pull_request.user.login == 'pre-commit-ci[bot]' ||
+       github.event.pull_request.user.login == 'dependabot[bot]')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Delete head branch
+        run: gh api -X DELETE "repos/${GITHUB_REPOSITORY}/git/refs/heads/${HEAD_REF}"
+        env:
+          HEAD_REF: ${{ github.event.pull_request.head.ref }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}