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/src/ports.rs

@@ -558,6 +558,8 @@ pub struct BackgroundTaskSpec {
     pub owner_agent_id: AgentId,
     /// Kind-specific task payload.
     pub kind: BackgroundTaskKind,
+    /// Optional parent background task.
+    pub parent_task_id: Option<TaskId>,
     /// Completion wake policy.
     pub wake_policy: BackgroundTaskWakePolicy,
     /// Optional command invocation for command-backed tasks.
@@ -2597,6 +2599,15 @@ pub trait BackgroundTaskStore: Send + Sync {
         agent_id: AgentId,
     ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError>;
 
+    /// Lists tasks owned by a parent background task.
+    ///
+    /// # Errors
+    /// [`BackgroundTaskPortError`] on store failure.
+    async fn list_children(
+        &self,
+        parent_task_id: TaskId,
+    ) -> Result<Vec<BackgroundTask>, BackgroundTaskPortError>;
+
     /// Lists terminal tasks whose completion has not been delivered yet.
     ///
     /// # Errors