ci: add dependency-review-action to scan PR dependency changes

Fails PRs that introduce dependencies with known high/critical
vulnerabilities; posts a summary comment on failure.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/dependency-review.yml

@@ -0,0 +1,19 @@
+name: Dependency Review
+
+on: pull_request
+
+permissions:
+  contents: read
+
+jobs:
+  dependency-review:
+    runs-on: ubicloud-standard-8
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/dependency-review-action@v4
+        with:
+          fail-on-severity: high
+          comment-summary-in-pr: on-failure