ci: Add OpenSSF Scorecard analysis

Run OpenSSF Scorecard on pushes to master, branch protection changes and
a weekly schedule, and upload its SARIF results to code scanning. It
reports supply-chain posture this repository cares about: pinned
actions, token permissions, branch protection and signed releases.

Actions are pinned to full commit SHAs, as in the existing workflows.
Permissions are read-only except the job's security-events (SARIF
upload) and id-token (publishing results to the public Scorecard API).

Scorecard runs only on the default branch, so this takes effect once
reviewability-v1 is merged into master.

Claude-Session: https://claude.ai/code/session_013gZvwvuocM7a7Ut4kiBFHw

Author: claude

.github/workflows/scorecard.yml

@@ -0,0 +1,37 @@
+name: Scorecard supply-chain security
+
+on:
+  branch_protection_rule:
+  schedule:
+    - cron: "29 4 * * 1"
+  push:
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  analysis:
+    name: Scorecard analysis
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      security-events: write
+      id-token: write
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          persist-credentials: false
+      - uses: ossf/scorecard-action@2d1146689b8cda280b9bc96326124645441f03bc # v2.4.4
+        with:
+          results_file: results.sarif
+          results_format: sarif
+          publish_results: true
+      - uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: SARIF file
+          path: results.sarif
+          retention-days: 5
+      - uses: github/codeql-action/upload-sarif@2892aa5e19bbd11bc0cff5427e3b750a04d9e3c2 # v4.38.2
+        with:
+          sarif_file: results.sarif