Show the resolved model per subagent in the task rows and completion notifications

Is your feature request related to a problem?

The terminal's running-task list shows each subagent's type and current activity (e.g. claude — Verifying exports…) but not which model it is running on. When an Agent call passes no model override, the subagent silently inherits the session model. From the UI there is no way to tell whether a background agent is burning the expensive parent model or a cheaper one — and the assistant's own self-report about it can be wrong, since models can't introspect their identity.

Describe the solution you'd like

Display the resolved model name on each agent row in the task list (e.g. claude · haiku — Verifying exports…), and include it in the agent completion summary. This makes model inheritance visible and lets users verify cost expectations (e.g. "no expensive-model subagents") at a glance instead of trusting the model's claim.

Environment: Claude Code CLI 2.1.233, macOS (darwin 24.6.0)