feat(workstate): dette de contrat DTO work-state (frontend)

Aligne le contrat DTO du panneau work-state (#5).

- ajout summary: string|null à BackgroundCompletion, mappé depuis task.summary
- affichage discret du résumé sur une tâche terminale
- suppression du chemin de merge top-level mort des background tasks

Tests verts : vitest workstate.test.tsx (21), suite complète (507),
npm run build (exit 0).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/workstate/ProjectWorkStatePanel.tsx

@@ -104,6 +104,11 @@ function formatTaskStatus(status: BackgroundCompletion["status"]): string {
   }
 }
 
+/** Terminal (finished) task states that carry a result summary (ticket #5). */
+function isTerminalTaskStatus(status: BackgroundCompletion["status"]): boolean {
+  return status === "completed" || status === "failed" || status === "cancelled";
+}
+
 function taskStatusClass(status: BackgroundCompletion["status"]): string {
   if (status === "running" || status === "pending") return "bg-warning/15 text-warning";
   if (status === "completed" || status === "delivered") return "bg-success/10 text-success";
@@ -407,6 +412,15 @@ function BackgroundTaskRow({
           {shortTicket(task.taskId)}
         </code>
       </div>
+      {task.summary && isTerminalTaskStatus(task.status) && (
+        <p
+          aria-label={`task ${shortTicket(task.taskId)} summary`}
+          title={task.summary}
+          className="mt-1 truncate pl-1 text-[11px] text-muted"
+        >
+          {task.summary}
+        </p>
+      )}
       {message && (
         <p role="alert" className="mt-1 text-xs text-danger">
           {message}

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

@@ -208,6 +208,7 @@ describe("ProjectWorkStatePanel", () => {
               kind: "command",
               state: "failed",
               exitCode: 1,
+              summary: "process failed",
               stderrTail: "boom",
               createdAtMs: 14,
               updatedAtMs: 15,
@@ -236,7 +237,10 @@ describe("ProjectWorkStatePanel", () => {
     // Optional fields omitted by the backend normalize to null (not undefined/NaN).
     expect(tasks[0]?.exitCode).toBeNull();
     expect(tasks[0]?.stdoutTail).toBeNull();
+    expect(tasks[0]?.summary).toBeNull();
     expect(tasks[2]?.stderrTail).toBe("boom");
+    // `summary` (ticket #5) is carried through when present.
+    expect(tasks[2]?.summary).toBe("process failed");
     // updatedAtMs (backend-emitted) is carried through for chronological ordering.
     expect(tasks.map((t) => t.updatedAtMs)).toEqual([11, 13, 15, 17]);
 
@@ -259,6 +263,72 @@ describe("ProjectWorkStatePanel", () => {
     expect(order.every((i) => i >= 0)).toBe(true);
   });
 
+  it("shows a terminal task summary and hides it when absent or non-terminal (#5)", async () => {
+    const workState = new MockWorkStateGateway();
+    workState._setProjectWorkState(PROJECT_ID, {
+      agents: [
+        {
+          agentId: "agent-summary",
+          name: "Reporter",
+          profileId: "codex",
+          busy: { state: "idle" },
+          backgroundTasks: [
+            {
+              taskId: "run-with-summary",
+              kind: "command",
+              state: "running",
+              summary: "should-not-appear-while-running",
+              createdAtMs: 40,
+              updatedAtMs: 40,
+            },
+            {
+              taskId: "done-with-summary",
+              kind: "command",
+              state: "completed",
+              summary: "Build succeeded in 12s",
+              createdAtMs: 30,
+              updatedAtMs: 30,
+            },
+            {
+              taskId: "fail-with-summary",
+              kind: "command",
+              state: "failed",
+              exitCode: 1,
+              summary: "Compile error: missing semicolon",
+              createdAtMs: 20,
+              updatedAtMs: 20,
+            },
+            {
+              taskId: "done-no-summary",
+              kind: "command",
+              state: "completed",
+              createdAtMs: 10,
+              updatedAtMs: 10,
+            },
+          ],
+        },
+      ],
+    });
+
+    renderPanel(workState);
+
+    const list = await screen.findByLabelText("Reporter background tasks");
+    // Terminal tasks with a summary render it discreetly.
+    expect(
+      within(list).getByLabelText("task done-wit summary").textContent,
+    ).toBe("Build succeeded in 12s");
+    expect(
+      within(list).getByText("Compile error: missing semicolon"),
+    ).toBeTruthy();
+    // A completed task without a summary shows no summary line.
+    expect(within(list).queryByLabelText("task done-no- summary")).toBeNull();
+    // A still-running task never shows its (non-terminal) summary.
+    expect(within(list).queryByLabelText("task run-with summary")).toBeNull();
+    expect(
+      within(list).queryByText("should-not-appear-while-running"),
+    ).toBeNull();
+  });
+
   it("renders a legacy agent without tickets", async () => {
     const workState = new MockWorkStateGateway();
     workState._setProjectWorkState(PROJECT_ID, {
@@ -280,6 +350,47 @@ describe("ProjectWorkStatePanel", () => {
     expect(screen.queryByLabelText("Legacy tickets")).toBeNull();
   });
 
+  it("ignores a stray top-level backgroundTasks array (nested tasks intact) (#5)", async () => {
+    // The backend nests tasks under their agent and emits no `ownerAgentId`; a
+    // project-root `backgroundTasks` array is not part of the contract and must
+    // not be merged in (dead path removed in ticket #5).
+    const workState = new MockWorkStateGateway();
+    workState._setProjectWorkState(PROJECT_ID, {
+      agents: [
+        {
+          agentId: "agent-nested",
+          name: "Nested",
+          profileId: "codex",
+          busy: { state: "idle" },
+          backgroundTasks: [
+            {
+              taskId: "nested-1",
+              kind: "command",
+              state: "completed",
+              createdAtMs: 1,
+              updatedAtMs: 2,
+            },
+          ],
+        },
+      ],
+      backgroundTasks: [
+        {
+          taskId: "orphan-top-level",
+          kind: "command",
+          state: "completed",
+          ownerAgentId: "agent-nested",
+          createdAtMs: 3,
+          updatedAtMs: 4,
+        },
+      ],
+    });
+
+    const state = await workState.getProjectWorkState(PROJECT_ID);
+    expect((state.agents[0]?.backgroundTasks ?? []).map((t) => t.taskId)).toEqual(
+      ["nested-1"],
+    );
+  });
+
   it("normalizes missing top-level work state arrays", async () => {
     const workState = new MockWorkStateGateway();
     workState._setProjectWorkState(PROJECT_ID, {});