feat: add rich run timeline backend

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

src/shared/contracts/sidecar.ts

@@ -152,6 +152,8 @@ export interface AgentActivityEvent {
   activityType: AgentActivityType;
   agentId?: string;
   agentName?: string;
+  sourceAgentId?: string;
+  sourceAgentName?: string;
   toolName?: string;
 }