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/shared/domain/event.ts

@@ -40,6 +40,10 @@ export interface SessionEventRecord {
   subagentEventKind?: SubagentEventKind;
   customAgentName?: string;
   customAgentDisplayName?: string;
+  customAgentDescription?: string;
+  subagentError?: string;
+  subagentToolCallId?: string;
+  subagentModel?: string;
 
   // Skill invoked fields
   skillName?: string;