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.

crates/application/src/workstate/mod.rs

@@ -221,6 +221,8 @@ pub struct AgentWorkState {
 pub struct AgentBackgroundTaskState {
     /// Stable task id.
     pub task_id: TaskId,
+    /// Optional parent background task id.
+    pub parent_task_id: Option<TaskId>,
     /// Kind discriminant.
     pub kind: BackgroundTaskKindLabel,
     /// Current lifecycle state.
@@ -609,6 +611,7 @@ impl From<BackgroundTask> for AgentBackgroundTaskState {
             flatten_background_rendezvous_context(&task.kind);
         Self {
             task_id: task.id,
+            parent_task_id: task.parent_task_id,
             kind: BackgroundTaskKindLabel::from(&task.kind),
             state: task.state,
             exit_code,