ci: Add automerge workflow

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"

.github/workflows/automerge.yml

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+# The pull_request_target workflow trigger is dangerous. Do not add unrelated logic to this workflow.
+# https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
+# https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target
+name: Auto-merge
+on: pull_request_target
+permissions:
+  pull-requests: write  # to approve the PR
+  contents: write  # to merge the PR
+jobs:
+  dependabot:
+    if: ${{ github.event.pull_request.user.login == 'dependabot[bot]' }}
+    runs-on: ubuntu-latest
+    steps:
+      - id: dependabot-metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+      - if: ${{ steps.dependabot-metadata.outputs.update-type != 'version-update:semver-major' || steps.dependabot-metadata.outputs.package-ecosystem == 'github_actions' }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr review --approve ${{ github.event.pull_request.html_url }}
+      - if: ${{ steps.dependabot-metadata.outputs.update-type != 'version-update:semver-major' || steps.dependabot-metadata.outputs.package-ecosystem == 'github_actions' }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr merge --auto --squash ${{ github.event.pull_request.html_url }}
+  precommit:
+    if: ${{ github.event.pull_request.user.login == 'pre-commit-ci[bot]' }}
+    runs-on: ubuntu-latest
+    steps:
+      - env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr review --approve ${{ github.event.pull_request.html_url }}
+      - env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr merge --auto --squash ${{ github.event.pull_request.html_url }}