The agent passed None for on_update, so tools (e.g. shell) never forwarded
their incremental output — the UI only saw the final result. Now:
- EventSink: Send + Sync (the forwarding closure must satisfy ToolUpdateSink)
- ToolUpdateSink becomes lifetime-parameterized (bare trait-object aliases
default to 'static, which forbade borrowing the caller's sink); all tool
impls updated to &ToolUpdateSink<'_>
- Agent::execute_single forwards each tool update as a ToolExecutionUpdate
event (Mutex provides interior mutability inside the Fn closure)
- TUI: running tools show the tail of the output live (newest lines), done
tools show the head + remaining-line hint
regression test: a streaming tool's on_update texts appear as agent events in
order.