security: add OpenSSF Scorecard analysis

.github/workflows/scorecard.yml

@@ -0,0 +1,45 @@
+name: OpenSSF Scorecard
+
+on:
+  push:
+    branches:
+      - main
+  schedule:
+    - cron: '43 5 * * 6'
+
+permissions: read-all
+
+jobs:
+  scorecard:
+    name: Scorecard
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    permissions:
+      contents: read
+      security-events: write
+      id-token: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Run OpenSSF Scorecard
+        uses: ossf/scorecard-action@2d1146689b8cda280b9bc96326124645441f03bc # v2.4.4
+        with:
+          results_file: results.sarif
+          results_format: sarif
+          publish_results: true
+
+      - name: Upload Scorecard SARIF artifact
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: scorecard-sarif
+          path: results.sarif
+          retention-days: 5
+
+      - name: Upload Scorecard results to code scanning
+        uses: github/codeql-action/upload-sarif@ff2f1c621b7f889edc0d3c761ac2e6a3f8cdb0dd # v4.37.7
+        with:
+          sarif_file: results.sarif

docs/contributing/security-guidelines.md

@@ -29,6 +29,14 @@ Do not dismiss a CodeQL alert merely because CI passes or exploitation has not b
 
 CodeQL complements tests and human review; it does not replace either.
 
+## OpenSSF Scorecard
+
+`.github/workflows/scorecard.yml` evaluates the public repository's supply-chain and repository-security posture on pushes to `main` and on a weekly schedule. It does not execute on pull requests because publishing Scorecard results requires an OIDC token; privileged result publication is therefore kept away from untrusted PR execution.
+
+Scorecard results are published to the OpenSSF Scorecard service, uploaded to GitHub code scanning, and retained as a short-lived SARIF workflow artifact for review/debugging. The workflow uses the official Scorecard Action and only approved result-handling actions pinned to reviewed commit SHAs.
+
+A Scorecard score is a diagnostic signal, not a release gate by itself. Maintainers should review individual checks and remediation guidance, prioritizing findings that align with the project's threat model, dependency policy, workflow permissions, branch/ruleset protections, and release controls. Do not make cosmetic changes solely to raise the aggregate score when they conflict with the project's actual architecture or security model.
+
 ## Vulnerabilities
 
 Do not open a public issue for a suspected vulnerability. Follow `SECURITY.md` and GitHub Private Vulnerability Reporting.