ci(security): add OpenSSF Scorecard workflow

.github/workflows/scorecard.yml

@@ -0,0 +1,48 @@
+name: OpenSSF Scorecard
+
+# Scores the repo's security posture (branch protection, pinned deps, token
+# permissions, etc.) and publishes to the OpenSSF public API so a badge can be
+# shown (badge is added in Plan E). REQUIRES the canonical repo to be PUBLIC —
+# see the maintainer checklist in docs/systems/security-scanning.md.
+
+on:
+  branch_protection_rule:
+  schedule:
+    - cron: '18 4 * * 2' # weekly Tuesday 04:18 UTC
+  push:
+    branches: [main]
+
+permissions: read-all
+
+jobs:
+  analysis:
+    name: Scorecard analysis
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write # upload SARIF
+      id-token: write        # publish_results OIDC attestation
+    steps:
+      - name: Harden the runner
+        uses: step-security/harden-runner@v2
+        with:
+          egress-policy: audit
+      - name: Checkout
+        uses: actions/checkout@v5
+        with:
+          persist-credentials: false
+      - name: Run Scorecard
+        uses: ossf/scorecard-action@v2
+        with:
+          results_file: results.sarif
+          results_format: sarif
+          publish_results: true
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: SARIF file
+          path: results.sarif
+          retention-days: 5
+      - name: Upload SARIF to code scanning
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: results.sarif