[deploy-plugin DPB-6] Two-phase config loader + deploy schema re-home + frozen legacy union

[rickylabs]

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

Two-phase loader: bootstrap-parse (identity + plugins + deploy.targets unstripped) → resolve adapter schemaLoaders → compose the target schema registry → full parse. Unknown target key ⇒ DeployTargetAdapterMissingError — the ONE deliberate behavior change (today unknown keys are silently stripped). DeployTargetBaseSchema re-homes to core ./config; @netscript/config keeps the loader seam + a frozen delegating legacy union. Includes resolveDeploymentEnvironment normalization (omitted Aspire env ⇒ production; bare + @production alias registrations rejected). (DP-2 §6; SF-10, SG-5.)

Acceptance

• gate: installed-custom-target, missing-adapter, malformed-config, loader-failure, and all-existing-key tests pass
• gate: unknown key errors with the target-add hint — never silently stripped
• gate: bare + @production alias registration rejected

Metadata

Dependencies: DPB-1
Delivery shape: config bootstrap contract