feat: livrable ticket #91 — notification fin BackgroundTask enrichie

- Backend: enrichissement HeadlessRendezvous avec requester/target/conversationId
- Frontend: toast 'Requester -> Target completed/...' explicite
- Clic ouvrant viewer de conversation pour visualiser l'échange

frontend/src/domain/index.ts

@@ -300,6 +300,9 @@ export type DomainEvent =
       taskId: string;
       agentId: string;
       state: string;
+      requesterAgentId?: string;
+      targetAgentId?: string;
+      conversationId?: string;
     }
   | {
       type: "agentInboxChanged";
@@ -608,6 +611,9 @@ export interface BackgroundCompletion {
   summary: string | null;
   stdoutTail: string | null;
   stderrTail: string | null;
+  requesterAgentId?: string;
+  targetAgentId?: string;
+  conversationId?: string;
   /** Last-update timestamp (epoch ms); chronological ordering key. */
   updatedAtMs: number;
 }