ci: delete redundant bot-branch-delete workflow; make cleanup explicit (#42)

Mirror the change from SrrDBAutomationToolkit#29:

- Add --delete-branch to both gh pr merge calls in auto-merge-bots.yml so
  bot PR branches are deleted explicitly at merge time.
- Remove the redundant delete-merged-bot-branches.yml on-close workflow,
  which raced with merge-time cleanup and only ever failed with
  "Reference does not exist".

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tablackburn

.github/workflows/auto-merge-bots.yml

@@ -19,7 +19,7 @@ jobs:
 
       - name: Auto-merge minor and patch updates
         if: steps.meta.outputs.update-type != 'version-update:semver-major'
-        run: gh pr merge --auto --squash "$PR_URL"
+        run: gh pr merge --auto --squash --delete-branch "$PR_URL"
         env:
           PR_URL: ${{ github.event.pull_request.html_url }}
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Auto-merge pre-commit.ci updates
-        run: gh pr merge --auto --squash "$PR_URL"
+        run: gh pr merge --auto --squash --delete-branch "$PR_URL"
         env:
           PR_URL: ${{ github.event.pull_request.html_url }}
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}