[codex] fix artifact registration payload

[hopeatina]

Summary

• stop sending top-level initiative_id on artifact create payloads for both the canonical client contract and the legacy fallback path
• keep initiative/workstream scope in artifact metadata and entity linkage instead of the rejected top-level field
• add regression coverage for the payload contract and update the lifecycle assertion that encoded the old shape

Why

The plugin-side artifact registration path still emitted initiative_id on artifact create requests. Current OrgX artifact writes reject that field for entity type artifact, which forced registrations into the local outbox fallback even when connectivity was healthy.

Impact

Artifact registration now stays on the normal upstream path for initiative-scoped artifacts instead of degrading to deferred local buffering.

Test plan

• npm run typecheck
• npm run build
• npm run test:file -- tests/artifacts/register-artifact-client-contract.test.mjs tests/http/autopilot-slice-lifecycle.test.mjs
• npm run test:hooks (known unrelated local baseline failure remains in tests/http/workspace-scope-live-surfaces.test.mjs, specifically workspace scope filters legacy live activity pages)