chore: restore self-hosted Renovate workflow

.github/workflows/renovate.yml

@@ -0,0 +1,27 @@
+name: Renovate
+
+on:
+  schedule:
+    - cron: "0 4,16 * * *"
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  pull-requests: write
+  issues: write
+
+jobs:
+  renovate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      # Run against this repo only, using the auto-provided GITHUB_TOKEN (no PAT to manage).
+      # GITHUB_TOKEN can't autodiscover, so the target repo is set explicitly. Trade-off: PRs that
+      # Renovate opens with GITHUB_TOKEN do not trigger this repo's other workflows, so dependency
+      # PRs arrive without CI until manually re-run. Swap to a GitHub App token if that becomes a pain.
+      - uses: renovatebot/github-action@v46.1.14
+        env:
+          RENOVATE_REPOSITORIES: ${{ github.repository }}
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          configurationFile: renovate.json