ci: implement automated maintenance via dependabot automerge

Add a dedicated workflow to enable GitHub's native auto-merge feature
for Dependabot PRs. This allows dependencies that pass CI to be
merged automatically, reducing manual maintenance overhead.

Author: gildesmarais

.github/workflows/dependabot-automerge.yml

@@ -0,0 +1,17 @@
+name: Dependabot Automerge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  automerge:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Enable automerge for Dependabot PRs
+        run: gh pr merge --auto --merge "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}