context: Document Agent Trace DWH identity contract

Record the separate append-oriented DWH destination boundary and distinguish deterministic logical identities from source-instance-scoped local row IDs. Update durable architecture links and mark the completed plan tasks so future ETL work can preserve idempotent re-ingestion without coupling live capture to warehouse concerns.\n\nPlan: agent-trace-dwh-schema-identity-contract\nTasks: T01, T02

Co-authored-by: SCE <sce@crocoder.dev>

Author: davidabram