feat(background-task): rendu live des tâches de fond — subscriber UI + canal IPC (#58)

Câble un canal attachable au flux de sortie d'une tâche de fond (runner
infrastructure + commande app-tauri + port/adaptateurs frontend) et le
panneau ProjectWorkStatePanel s'y abonne pour un rendu live au lieu d'un
état figé au dernier snapshot.

Validations obtenues avant commit :
- cargo test -p infrastructure --test background_task_runner : vert
- cargo check -p backend -p app-tauri : vert
- npx vitest run src/features/workstate/workstate.test.tsx : vert
- npm run typecheck : vert
- verdict QA #58 : vert

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/workstate/workstate.test.tsx

@@ -336,6 +336,64 @@ describe("ProjectWorkStatePanel", () => {
     ).toBeNull();
   });
 
+  it("attaches, repaints, streams, and detaches live background task output", async () => {
+    const workState = new MockWorkStateGateway();
+    workState._setProjectWorkState(PROJECT_ID, {
+      agents: [
+        {
+          agentId: "agent-live-task",
+          name: "Runner",
+          profileId: "codex",
+          busy: { state: "idle" },
+          backgroundTasks: [
+            {
+              taskId: "task-live-output",
+              kind: "command",
+              state: "running",
+              createdAtMs: 10,
+              updatedAtMs: 10,
+            },
+          ],
+        },
+      ],
+    });
+    workState._setBackgroundTaskAttachment("task-live-output", {
+      scrollback: "already printed\n",
+      live: true,
+    });
+    const attachSpy = vi.spyOn(workState, "attachBackgroundTask");
+
+    const view = renderPanel(workState);
+    const list = await screen.findByLabelText("Runner background tasks");
+
+    fireEvent.click(within(list).getByRole("button", { name: "Live" }));
+
+    expect(attachSpy).toHaveBeenCalledWith("task-live-output", expect.any(Function));
+    expect(
+      (await within(list).findByLabelText("task task-liv live output")).textContent,
+    ).toContain("already printed");
+    expect(workState._backgroundTaskSubscriberCount("task-live-output")).toBe(1);
+
+    workState._emitBackgroundTaskOutput("task-live-output", "new line\n");
+    await waitFor(() =>
+      expect(
+        within(list).getByLabelText("task task-liv live output").textContent,
+      ).toContain("new line"),
+    );
+
+    fireEvent.click(within(list).getByRole("button", { name: "Detach" }));
+    await waitFor(() =>
+      expect(workState._backgroundTaskSubscriberCount("task-live-output")).toBe(0),
+    );
+    expect(within(list).queryByLabelText("task task-liv live output")).toBeNull();
+
+    fireEvent.click(within(list).getByRole("button", { name: "Live" }));
+    await within(list).findByLabelText("task task-liv live output");
+    expect(workState._backgroundTaskSubscriberCount("task-live-output")).toBe(1);
+    view.unmount();
+    expect(workState._backgroundTaskSubscriberCount("task-live-output")).toBe(0);
+  });
+
   it("renders a legacy agent without tickets", async () => {
     const workState = new MockWorkStateGateway();
     workState._setProjectWorkState(PROJECT_ID, {