[deploy-plugin DPB-25] Vercel probe + deploy-vercel (Build Output API) + Story 4

[rickylabs]

Part of #892 (RFC #891; corpus .llm/runs/plan-deploy-plugin--seed/ r5).

Probe first (minimal service through .vercel/output → live deploy → HTTP conformance), then the adapter: Build Output API v3 emission + vercel deploy --prebuilt; Node runtime default (honest T3), community vercel-deno opt-in with a version-lag warning; Story-4 scaffold assets. Anti-scope: marketplace backings (Upstash/Neon/QStash) are leaf cards (DP-3 §6, L-7).

Acceptance

• gate: .vc-config.json emission validated by the live probe before adapter claims
• gate: conformance cells for claimed verdicts

Metadata

Dependencies: DPB-5, DPB-17
Delivery shape: probe-gated adapter