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/domain/tests/background_task.rs

@@ -61,6 +61,20 @@ fn rendezvous_link_roundtrips_on_background_task() {
     assert_eq!(decoded.rendezvous, Some(link));
 }
 
+#[test]
+fn parent_task_link_roundtrips_and_rejects_self_parent() {
+    let task = command_task().with_parent_task(task_id(9)).unwrap();
+
+    let json = serde_json::to_string(&task).unwrap();
+    let decoded: BackgroundTask = serde_json::from_str(&json).unwrap();
+
+    assert_eq!(decoded.parent_task_id, Some(task_id(9)));
+    assert_eq!(
+        command_task().with_parent_task(task_id(1)).unwrap_err(),
+        BackgroundTaskError::SelfParent
+    );
+}
+
 fn failure_result(at: u64) -> BackgroundTaskResult {
     BackgroundTaskResult::Failure {
         finished_at_ms: at,