fix(runtime): cancel en cascade des tasks filles — stop agent / cancel parent (#200, QA verte)

Cancel transitif, idempotent et ownership-aware sur le graphe parent→enfants des background tasks:

- domain/background_task + ports: modèle parent/enfant (parent_task_id, rejet du self-parent), index children_by_parent, traversal des descendants en largeur.

- application/background + infrastructure/store: cascade cancel (annule les descendants running/open, laisse les terminaux), idempotente face aux double-cancel et ordres de terminaison non déterministes; respect de l'ownership (parent_task_link).

- orchestrator/service: stop agent → cascade cancel des tasks lancées par cet agent (relatesTo #196); cancel task parent → cascade de ses sous-tasks (dependsOn #199). Refactor unifiant le chemin de cancel, sans sémantique #196/#199 regreffée (tests hérités verts).

- frontend: projection parentTaskId (workstate normalization/domain/services).

Tests QA verts: domain background_task, application background_task_cancel (cascades_background_task_subtree, cascades_to_running_child, cascades_to_open_descendants_and_leaves_terminal_child, parent_task_link_roundtrips_and_rejects_self_parent), orchestrator_service, workstate_actions, workstate.test.tsx.

frontend/src/adapters/workStateNormalization.ts

@@ -113,10 +113,12 @@ function normalizeBackgroundTask(
   const conversationId = optionalString(
     task.conversationId ?? task.conversation_id,
   );
+  const parentTaskId = optionalString(task.parentTaskId ?? task.parent_task_id);
   return {
     taskId: stringValue(task.taskId, `legacy-task-${index}`),
     ownerAgentId: stringValue(task.ownerAgentId, fallbackAgentId),
     projectId: stringValue(task.projectId),
+    ...(parentTaskId ? { parentTaskId } : {}),
     kind: stringValue(task.kind, "background"),
     status: normalizeBackgroundStatus(task.status ?? task.state),
     exitCode: nullableNumber(task.exitCode),