test: add encrypted secrets test to overlayfs workflow

- Generate random encryption key and store in GitHub Secrets via gh CLI
- Encrypt test message with GPG and commit to repo
- Decrypt in workflow using key from secrets and echo result
- Demonstrates encrypted secrets approach for SSH keys

.github/secrets/test-message.gpg

@@ -0,0 +1 @@
+<EFBFBD>

.github/workflows/test-overlayfs-delta.yml

@@ -9,6 +9,31 @@ jobs:
   test-overlayfs:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Test encrypted secrets (decrypt test message)
+        run: |
+          echo "========================================"
+          echo "TESTING ENCRYPTED SECRETS"
+          echo "========================================"
+          echo ""
+          echo "Decrypting test message from .github/secrets/test-message.gpg"
+          echo "Using encryption key from GitHub Secrets..."
+          echo ""
+
+          # Decrypt using key from GitHub Secrets
+          echo "${{ secrets.TEST_ENCRYPTION_KEY }}" | \
+            gpg --batch --yes --passphrase-fd 0 \
+            --decrypt .github/secrets/test-message.gpg
+
+          echo ""
+          echo "========================================"
+          echo "If you see the success message above,"
+          echo "then encrypted secrets work! 🎉"
+          echo "========================================"
+          echo ""
+
       - name: Setup OverlayFS layers
         run: |
           echo "=== Creating directory structure ==="