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/plugin/mod.rs

@@ -2011,6 +2011,7 @@ impl PluginCommandTasks {
                 command,
                 wake_policy,
                 rendezvous: None,
+                parent_task_id: None,
                 deadline_ms: input.deadline_ms,
             })
             .await
@@ -4384,6 +4385,20 @@ mod tests {
                 .collect())
         }
 
+        async fn list_children(
+            &self,
+            parent_task_id: TaskId,
+        ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError> {
+            Ok(self
+                .tasks
+                .lock()
+                .unwrap()
+                .values()
+                .filter(|task| task.parent_task_id == Some(parent_task_id))
+                .cloned()
+                .collect())
+        }
+
         async fn list_undelivered_completions(
             &self,
         ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError> {