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/backend/src/lib.rs

@@ -576,6 +576,34 @@ impl BackgroundTaskStore for AppBackgroundTaskStore {
         Ok(tasks)
     }
 
+    async fn list_children(
+        &self,
+        parent_task_id: TaskId,
+    ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError> {
+        let project_ids = if let Some(project_id) = self.project_for_task(parent_task_id).await? {
+            vec![project_id]
+        } else {
+            self.projects
+                .list_projects()
+                .await
+                .map_err(|err| BackgroundTaskPortError::Store(err.to_string()))?
+                .into_iter()
+                .map(|project| project.id)
+                .collect()
+        };
+
+        let mut tasks = Vec::new();
+        for project_id in project_ids {
+            let store = self.store_for_project(project_id).await?;
+            tasks.extend(store.list_children(parent_task_id).await?);
+        }
+        for task in &tasks {
+            self.remember(task.id, task.project_id);
+        }
+        tasks.sort_by_key(|task| (task.created_at_ms, task.id));
+        Ok(tasks)
+    }
+
     async fn list_undelivered_completions(
         &self,
     ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError> {