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/adapters/workStateNormalization.ts

@@ -100,6 +100,15 @@ function normalizeBackgroundTask(
   fallbackAgentId: string,
 ): BackgroundCompletion {
   const task = isRecord(value) ? value : {};
+  const requesterAgentId = optionalString(
+    task.requesterAgentId ?? task.requester_agent_id,
+  );
+  const targetAgentId = optionalString(
+    task.targetAgentId ?? task.target_agent_id,
+  );
+  const conversationId = optionalString(
+    task.conversationId ?? task.conversation_id,
+  );
   return {
     taskId: stringValue(task.taskId, `legacy-task-${index}`),
     ownerAgentId: stringValue(task.ownerAgentId, fallbackAgentId),
@@ -110,6 +119,9 @@ function normalizeBackgroundTask(
     summary: nullableString(task.summary),
     stdoutTail: nullableString(task.stdoutTail),
     stderrTail: nullableString(task.stderrTail),
+    ...(requesterAgentId ? { requesterAgentId } : {}),
+    ...(targetAgentId ? { targetAgentId } : {}),
+    ...(conversationId ? { conversationId } : {}),
     updatedAtMs: numberValue(task.updatedAtMs),
   };
 }