Incrementally transforms raw telemetry events into typed display items.

Handles event pairing (llm_stop + eval_stop), filtering of internal events, sub-agent grouping, and collapsing of redundant eval_and_complete + return/done pairs.

Item types

• {:message, data} - user message to the agent
• {:human_response, data} - user response to HumanTool.ask
• {:exception, data} - fatal error during message handling
• {:step, data} - one LLM decision, optionally paired with eval outcome
• {:eval_error, data} - standalone eval failure (unmatched with llm_stop)
• {:subagent, name, items} - collapsible group of sub-agent events
• {:unknown, data} - unrecognized event type