SHA-pin GitHub Actions and upgrade deprecated checkout versions

- Upgrade actions/checkout from v2/v3 to SHA-pinned v4
- SHA-pin all unshelled action tags (pages, CodeQL, scorecard,
  rust-cache, upload/download-artifact, setup-node, cache)
- Standardise scorecard-action to v2.4.0
- Fix setup-node@v6 → SHA-pinned v4

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/security-scan.yml

@@ -29,7 +29,7 @@ jobs:
 
       - name: Cache panic-attacker binary
         id: cache-pa
-        uses: actions/cache@v4
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
         with:
           path: ~/.cargo/bin/panic-attacker
           key: panic-attacker-${{ runner.os }}