ci: auto-merge low-risk Dependabot PRs

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

@@ -0,0 +1,41 @@
+name: Dependabot auto-merge
+
+on:
+  pull_request_target:
+    branches: [master]
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    name: Auto-merge low-risk Dependabot PRs
+    if: github.event.pull_request.user.login == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fetch Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@25dd0e34f4fe68f24cc83900b1fe3fe149efef98 # v3
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      # Arming auto-merge is the whole job, and the merge still waits for CI:
+      # `--auto` queues it behind the required checks rather than merging now.
+      # There is no approve step on purpose. GITHUB_TOKEN cannot approve a pull
+      # request ("Allow GitHub Actions to create and approve pull requests" is
+      # off org-wide), so an approve step would fail on every single bump, and a
+      # check that is always red teaches you to stop reading red checks.
+      #
+      # Scope is deliberately narrow: workflow action bumps only, minor and
+      # patch. A pub dependency reaches the package's own resolution graph and a
+      # major bump is a breaking change by declaration, so both keep a human.
+      - name: Arm auto-merge for GH Actions minor + patch
+        if: |
+          steps.metadata.outputs.package-ecosystem == 'github_actions' &&
+          (steps.metadata.outputs.update-type == 'version-update:semver-minor' ||
+           steps.metadata.outputs.update-type == 'version-update:semver-patch')
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr merge --auto --squash "$PR_URL"