windows-20260905-100718

fix(core): surface inter-agent messages in thread views

Incoming agent_message items were only emitted as raw response items,
which thread transcripts do not render, so a spawned subagent's thread
showed only its own work and not the task the parent sent. Emit a
UserMessage turn item with the flattened envelope plus payload when
recording inter-agent communication, and project persisted
agent_message response items to the same user message item during
rollout replay so resumed threads show it too.