security: harden OpenSSF Scorecard compliance

- Pin all GitHub Actions by SHA hash (Pinned-Dependencies)
- Add top-level permissions: read-all (Token-Permissions)
- Add explicit job-level permissions (least privilege)
- Add OpenSSF Scorecard workflow with weekly schedule
- Publish scorecard results to scorecard.dev and GitHub Security tab

.github/workflows/scorecard.yml

@@ -0,0 +1,35 @@
+name: OpenSSF Scorecard
+
+on:
+  push:
+    branches: [main]
+  schedule:
+    - cron: '0 6 * * 1'  # every Monday at 06:00 UTC
+
+permissions: read-all
+
+jobs:
+  analysis:
+    name: Scorecard analysis
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write
+      id-token: write
+
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        with:
+          persist-credentials: false
+
+      - name: Run OpenSSF Scorecard
+        uses: ossf/scorecard-action@99c09fe975337306107572b4fdf4db224cf8e2f2 # v2.4.3
+        with:
+          results_file: results.sarif
+          results_format: sarif
+          publish_results: true
+
+      - name: Upload Scorecard results to GitHub Security tab
+        uses: github/codeql-action/upload-sarif@a60c4df7a135c7317c1e9ddf9b5a9b07a910dda9 # v4
+        with:
+          sarif_file: results.sarif
+          category: scorecard