feat: show sub-agent activity cards in chat stream

Display compact status cards in the chat transcript showing each
sub-agent's name, current activity (Thinking, Using grep, etc.),
and elapsed time. Cards appear while sub-agents are running and
clear when the session goes idle.

- Extend SessionEventRecord with description, error, toolCallId,
  and model fields for subagent events
- Forward additional subagent fields in handleTurnScopedEvent
- Add subagentTracker reducer to derive active subagent state from
  session events, including agent-activity correlation
- Create SubagentActivityCard component with spinner, status icon,
  agent name, activity label, and live elapsed timer
- Wire activeSubagents state in App.tsx and pass to ChatPane
- Add 16 unit tests for the subagent tracker

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/AryxAppService.ts

@@ -1827,6 +1827,10 @@ export class AryxAppService extends EventEmitter<AppServiceEvents> {
           subagentEventKind: event.eventKind,
           customAgentName: event.customAgentName,
           customAgentDisplayName: event.customAgentDisplayName,
+          customAgentDescription: event.customAgentDescription,
+          subagentError: event.error,
+          subagentToolCallId: event.toolCallId,
+          subagentModel: event.model,
         });
         return;
       case 'skill-invoked':