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/domain/index.ts

@@ -330,6 +330,8 @@ export interface BackgroundCompletion {
   kind: string;
   status: "running" | "completed" | "failed" | "cancelled" | "pending" | "delivered";
   exitCode: number | null;
+  /** Human-readable summary / error / reason of a terminal result (ticket #5). */
+  summary: string | null;
   stdoutTail: string | null;
   stderrTail: string | null;
   /** Last-update timestamp (epoch ms); chronological ordering key. */