[deploy-plugin DPB-18] Story-0 scaffold.runtime E2E (install, target add, plan)

[rickylabs]

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

Registered scaffold.runtime case: plugin install deploy → deploy target add deno-deploy → deploy deno-deploy plan on a generated workspace. Anti-scope: live platform deploys stay on probe lanes.

Acceptance

• gate: the case is registered in runtime-gates and green in the merge-readiness suite

Metadata

Dependencies: DPB-16, DPB-17
Delivery shape: e2e registration