fix(runtime): preserve child agent report identity

[yanpgwang]

Summary

• preserve from_agent_name and from_session_thread_id when projecting agent.thread_message_received into model context
• add runtime-owned coordinator guidance for asynchronous reports, persistent follow-ups, and dependent delegation
• keep rich cross-thread content blocks intact inside a structured internal-message envelope
• correct the multi-agent guide so dependent review is delegated only after its prerequisite report arrives

Problem

A child Thread report woke the primary Thread correctly, but the model projection discarded its sender metadata and presented only the report content as a plain user-role message. The coordinator could therefore mistake a worker report for content authored by the Session user and fail to continue orchestration.

Compatibility basis

The public Managed Agents multi-agent contract defines agent.thread_message_received as a report or question sent to the coordinator and includes from_session_thread_id, from_agent_name, and content: https://platform.claude.com/docs/en/managed-agents/multiagent-orchestration#primary-thread-events

The public HTTP event shape, persistence schema, and Temporal wakeup path are unchanged. The change affects only provider-facing context projection and private coordinator system guidance. Existing persisted cross-Thread events are projected through the new envelope on their next model turn, including legacy events whose sender metadata is absent.

Validation

• make verify
• make docs-check
• make local-config
• make test-service
• documentation typecheck and production build