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/adapters/workStateNormalization.ts

@@ -107,6 +107,7 @@ function normalizeBackgroundTask(
     kind: stringValue(task.kind, "background"),
     status: normalizeBackgroundStatus(task.status ?? task.state),
     exitCode: nullableNumber(task.exitCode),
+    summary: nullableString(task.summary),
     stdoutTail: nullableString(task.stdoutTail),
     stderrTail: nullableString(task.stderrTail),
     updatedAtMs: numberValue(task.updatedAtMs),
@@ -215,17 +216,11 @@ function normalizeConversation(
 export function normalizeProjectWorkState(value: unknown): ProjectWorkState {
   const state = isRecord(value) ? value : {};
   const agents = arrayValue(state.agents).map(normalizeAgent);
-  const topLevelTasks = arrayValue(
-    state.backgroundTasks ?? state.backgroundCompletions,
-  ).map((task, index) => normalizeBackgroundTask(task, index, ""));
-  for (const task of topLevelTasks) {
-    const owner = agents.find((agent) => agent.agentId === task.ownerAgentId);
-    if (!owner) continue;
-    owner.backgroundTasks ??= [];
-    if (!owner.backgroundTasks.some((item) => item.taskId === task.taskId)) {
-      owner.backgroundTasks.push(task);
-    }
-  }
+  // Background tasks are always emitted nested under their owning agent
+  // (`agent.backgroundTasks`) and the backend DTO carries no `ownerAgentId`, so a
+  // top-level merge keyed on `task.ownerAgentId` is unreachable for this contract
+  // and has been removed (ticket #5). The per-agent `ownerAgentId = agentId`
+  // fallback in `normalizeBackgroundTask` is what nested tasks rely on.
   const topLevelInbox = arrayValue(state.inboxItems).map((item, index) =>
     normalizeInboxItem(item, index, ""),
   );