Re-verified against the tui_gateway emitters:
- SubagentEventPayload.cost_usd / .iteration: not in
tool_progress.py::_SUBAGENT_FIELDS, never emitted → removed; the TUI's
turnController no longer copies them (its SubagentProgress keeps the
fields for spawn-history persistence).
- SubagentEventPayload.child_session_id: emitted (in _SUBAGENT_FIELDS, read
by agent_callbacks.py::_mirror_subagent_to_child) but untyped → added.
- ToolCompletePayload.error: _on_tool_complete never sets it → removed;
the TUI's completeTool drops its dead `error` parameter and renders the
trail line as non-error (which is what it always did on the wire).
- ToolStartPayload.todos: not on the wire either, but the TUI handler and
its fixtures exercise recordTodos from tool.start; kept with a comment
saying so rather than churning the handler.
- MessageCompletePayload.failure_reason: prompt_turn.py passes
result.get("failure_reason") through → `string | null`.
c1e0fd83f9