fix(ci): avoid empty pnpm cache cleanup

[Codevena]

What changed

• disable actions/setup-node pnpm caching only in the dependency-free security job
• add a regression test that keeps the audit step present and rejects that invalid cache configuration

Root cause

The security job intentionally runs pnpm audit without installing dependencies. Its audit and Gitleaks steps passed, but actions/setup-node failed during post-job cleanup because no pnpm store path existed to cache.

Validation

• regression test observed red before the workflow fix and green afterward
• actionlint .github/workflows/ci.yml
• pnpm lint
• pnpm typecheck
• pnpm test — 443/443
• pnpm build — 48/48 pages
• two independent diff reviews: PASS with no findings

Production context

AgoraHub v0.2.0-alpha is already healthy on Coolify from commit b20d821; this follow-up fixes only the false-negative CI cleanup failure.