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/plugins/runtime/services.ts

@@ -141,6 +141,7 @@ export interface BackgroundTaskStatus {
   taskId: string;
   ownerAgentId: string;
   projectId: string;
+  parentTaskId?: string;
   kind: string;
   status:
     | "pending"
@@ -447,6 +448,7 @@ function toBackgroundTaskStatus(task: BackgroundCompletion): BackgroundTaskStatu
     taskId: task.taskId,
     ownerAgentId: task.ownerAgentId,
     projectId: task.projectId,
+    ...(task.parentTaskId ? { parentTaskId: task.parentTaskId } : {}),
     kind: task.kind,
     status: task.status,
     exitCode: task.exitCode,