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/renderer/components/ChatPane.tsx

@@ -10,12 +10,14 @@ import { UserInputBanner } from '@renderer/components/chat/UserInputBanner';
 import { InlineApprovalPill, InlineModelPill, InlineTerminalPill, InlineThinkingPill, InlineToolsPill } from '@renderer/components/chat/InlinePills';
 import { InlinePromptPill } from '@renderer/components/chat/InlinePromptPill';
 import { ThinkingDots } from '@renderer/components/chat/ThinkingDots';
+import { SubagentActivityList } from '@renderer/components/chat/SubagentActivityCard';
 import { getAssistantMessagePhase } from '@renderer/lib/messagePhase';
 import type { ApprovalDecision } from '@shared/domain/approval';
 import type { InteractionMode, MessageMode } from '@shared/contracts/sidecar';
 import type { ChatMessageAttachment } from '@shared/domain/attachment';
 import { getAttachmentDisplayName, isImageAttachment } from '@shared/domain/attachment';
 import type { SessionUsageState } from '@renderer/lib/sessionActivity';
+import type { ActiveSubagent } from '@renderer/lib/subagentTracker';
 import {
   findModel,
   getSupportedReasoningEfforts,
@@ -44,6 +46,7 @@ interface ChatPaneProps {
   mcpProbingServerIds?: string[];
   runtimeTools?: ReadonlyArray<RuntimeToolDefinition>;
   sessionUsage?: SessionUsageState;
+  activeSubagents?: ReadonlyArray<ActiveSubagent>;
   terminalOpen?: boolean;
   terminalRunning?: boolean;
   onSend: (content: string, attachments?: ChatMessageAttachment[], messageMode?: MessageMode) => Promise<void>;
@@ -72,6 +75,7 @@ export function ChatPane({
   mcpProbingServerIds,
   runtimeTools,
   sessionUsage,
+  activeSubagents,
   terminalOpen,
   terminalRunning,
   onSend,
@@ -409,6 +413,11 @@ export function ChatPane({
                 );
               })}
             </div>
+            {activeSubagents && activeSubagents.length > 0 && (
+              <div className="px-6 py-1">
+                <SubagentActivityList subagents={activeSubagents} />
+              </div>
+            )}
           </div>
         )}
       </div>