ci: auto-merge Dependabot patch/minor updates

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

@@ -0,0 +1,35 @@
+name: dependabot-auto-merge
+
+# Queue Dependabot patch and minor updates for auto-merge. GitHub merges them
+# only after the branch's required checks pass. Major updates are left for a
+# human to review.
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    if: github.event.pull_request.user.login == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - id: meta
+        uses: dependabot/fetch-metadata@25dd0e34f4fe68f24cc83900b1fe3fe149efef98 # v3.1.0
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Enable auto-merge (patch and minor only)
+        if: steps.meta.outputs.update-type != 'version-update:semver-major'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+        run: gh pr merge --auto --squash "$PR_URL"
+
+      - name: Leave major updates for review
+        if: steps.meta.outputs.update-type == 'version-update:semver-major'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+        run: gh pr comment "$PR_URL" --body "Major version update. Not auto-merged; please review the changelog."