Vercel AI SDK harnesses add native subscription auth

Bridging developer subscription tiers into agent harnesses without leaking credentials solves a major friction point in coding agent adoption. Developers and teams can now leverage existing paid seats in automated pipelines without managing secondary API keys or paying twice for inference. Keeping token refreshes on the host and injecting active tokens into outbound requests ensures sandboxed agent processes never hold real secrets, establishing a strong security model for untrusted execution environments. Standardizing on the HarnessAgent interface enables applications to hot-swap between competing coding agents without rewriting orchestration code, while broad day-one harness support positions Vercel AI SDK as a central orchestration layer for multi-agent engineering.