ci: add Renovate config lint workflow

Validate renovate.json on changes via renovate-config-validator
to catch typos and schema regressions before merge.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vlsi

.github/workflows/renovate-config-lint.yaml

@@ -0,0 +1,27 @@
+name: Validate Renovate Config
+
+on:
+  push:
+    paths:
+      - renovate.json
+      - .github/workflows/renovate-config-lint.yaml
+  pull_request:
+    paths:
+      - renovate.json
+      - .github/workflows/renovate-config-lint.yaml
+
+# Declare default permissions as read-only.
+permissions: read-all
+
+jobs:
+  validate-renovate-config:
+    name: Validate renovate.json
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+        with:
+          persist-credentials: false
+
+      - name: Validate renovate.json
+        run: docker run --rm -v "$PWD":/work -w /work renovate/renovate:latest renovate-config-validator renovate.json