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/features/workstate/workstate.test.tsx

@@ -180,6 +180,7 @@ describe("ProjectWorkStatePanel", () => {
     // Mirrors AgentBackgroundTaskStateDto (crates/app-tauri/src/dto.rs): camelCase,
     // `state` (not `status`), optional exitCode/summary/stdoutTail/stderrTail omitted
     // when absent, createdAtMs/updatedAtMs present, NO ownerAgentId/projectId/finishedAtMs.
+    // Headless rendezvous tasks may carry requester/target/conversation context.
     const workState = new MockWorkStateGateway();
     workState._setProjectWorkState(PROJECT_ID, {
       agents: [
@@ -200,6 +201,9 @@ describe("ProjectWorkStatePanel", () => {
               taskId: "task-queued-1",
               kind: "headlessRendezvous",
               state: "queued",
+              requesterAgentId: "agent-main",
+              targetAgentId: "agent-bg",
+              conversationId: "conversation-rdv",
               createdAtMs: 12,
               updatedAtMs: 13,
             },
@@ -241,6 +245,9 @@ describe("ProjectWorkStatePanel", () => {
     expect(tasks[2]?.stderrTail).toBe("boom");
     // `summary` (ticket #5) is carried through when present.
     expect(tasks[2]?.summary).toBe("process failed");
+    expect(tasks[1]?.requesterAgentId).toBe("agent-main");
+    expect(tasks[1]?.targetAgentId).toBe("agent-bg");
+    expect(tasks[1]?.conversationId).toBe("conversation-rdv");
     // updatedAtMs (backend-emitted) is carried through for chronological ordering.
     expect(tasks.map((t) => t.updatedAtMs)).toEqual([11, 13, 15, 17]);