[deploy-plugin DPB-11] Legacy/config compatibility gate

[rickylabs]

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

First-class compatibility verification: golden argv/help/exit-code tests for every legacy verb; unknown-target error paths; artifact-side-effect goldens for legacy vs canonical Aspire paths. Anti-scope: no behavior fixes here — findings route to the owning card (SF-9/SF-10, SG-7).

Acceptance

• gate: every documented deploy invocation covered by a golden
• gate: canonical-vs-legacy divergence is exactly the enumerated set (DP-6 §3)

Metadata

Dependencies: DPB-6, DPB-7, DPB-8
Delivery shape: compat test suite