[deploy-plugin DPB-3] Empty duplicate-rejecting core registry + CLI compatibility composition root

[rickylabs]

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

createDeployTargetRegistry(entries = []) in core: registry port, key/error types, DeployTargetCollisionError (duplicate rejection is NEW — the shipped registry is register-or-replace), composition-root-only replaceForCompatibility for the W1 shim. DEFAULT_DEPLOY_TARGETS is deleted as a core concept; defaults stay in the CLI composition root. Anti-scope: core never imports an adapter (SF-1/SF-13).

Acceptance

• gate: constructor + generated-registry duplicate tests reject with both owners named
• gate: environment-qualified keys and deterministic ordering tested
• gate: e2e:cli green via the compat composition root

Metadata

Dependencies: DPB-1
Delivery shape: registry extraction + NEW rejection behavior