ci: add Dependabot auto-merge for patch updates

Auto-approve and squash-merge Dependabot PRs when the update type
is semver-patch. Minor and major bumps still require manual review.
Also enables the repository auto-merge setting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: etoyama

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

@@ -0,0 +1,25 @@
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    if: github.actor == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fetch Dependabot metadata
+        id: meta
+        uses: dependabot/fetch-metadata@v2
+
+      - name: Auto-approve and merge patch updates
+        if: steps.meta.outputs.update-type == 'version-update:semver-patch'
+        run: |
+          gh pr review "$PR_URL" --approve
+          gh pr merge "$PR_URL" --squash --auto
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}