ci(security): add pnpm audit signatures step to validate job

Catches tampered package signatures (compromised maintainer
supply-chain attack) before they reach CI artifacts.

.github/workflows/ci.yml

@@ -51,6 +51,8 @@ jobs:
           node-version: 22
           cache: pnpm
       - run: pnpm install --frozen-lockfile
+      - name: Audit package signatures
+        run: pnpm audit signatures --audit-level=high
       - name: Socket supply-chain scan
         if: github.event_name == 'pull_request'
         run: |