chore: add dependabot config and auto-merge workflow (#462)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/dependabot.yml

@@ -0,0 +1,24 @@
+name: Dependabot
+
+on:
+  pull_request:
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+    - name: Dependabot metadata
+      id: metadata
+      uses: dependabot/fetch-metadata@v2
+
+    - name: Enable auto-merge for Dependabot PRs
+      if: ${{ steps.metadata.outputs.update-type != 'version-update:semver-major' }}
+      run: gh pr merge --auto --merge "$PR_URL"
+      env:
+        PR_URL: ${{ github.event.pull_request.html_url }}
+        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}