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}