[deploy-plugin DPB-22] Container scaffold story (Story 3) + artifact goldens

[rickylabs]

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

target add fly|koyeb|sevalla|coolify|dokploy scaffold assets: generated Dockerfile (+ compose for local parity), workflows, env-name contracts. Anti-scope: platform add-on catalog rows only where leaf backings exist.

Acceptance

• gate: goldens for every emitted artifact
• gate: emitted workflows reference only documented secret/OIDC contracts

Metadata

Dependencies: DPB-17, DPB-20
Delivery shape: scaffold assets