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/dto.rs

@@ -3638,6 +3638,9 @@ impl From<AgentTicketState> for AgentTicketStateDto {
 pub struct AgentBackgroundTaskStateDto {
     /// Stable task id (UUID string).
     pub task_id: String,
+    /// Optional parent background task id (UUID string).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_task_id: Option<String>,
     /// Kind discriminant.
     pub kind: String,
     /// Lifecycle state.
@@ -3673,6 +3676,7 @@ impl From<AgentBackgroundTaskState> for AgentBackgroundTaskStateDto {
     fn from(task: AgentBackgroundTaskState) -> Self {
         Self {
             task_id: task.task_id.to_string(),
+            parent_task_id: task.parent_task_id.map(|id| id.to_string()),
             kind: background_kind_label_from_work_state(task.kind).to_owned(),
             state: background_state_label(task.state).to_owned(),
             exit_code: task.exit_code,
@@ -5102,6 +5106,9 @@ pub struct BackgroundTaskDto {
     pub owner_agent_id: String,
     /// Owning project id (UUID string).
     pub project_id: String,
+    /// Optional parent background task id (UUID string).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_task_id: Option<String>,
     /// Kind discriminant (`"command"`, `"headlessRendezvous"`, `"sessionResume"`,
     /// `"maintenance"`).
     pub kind: String,
@@ -5212,6 +5219,7 @@ impl From<BackgroundTask> for BackgroundTaskDto {
             task_id: task.id.to_string(),
             owner_agent_id: task.owner_agent_id.to_string(),
             project_id: task.project_id.to_string(),
+            parent_task_id: task.parent_task_id.map(|id| id.to_string()),
             kind: background_kind_label(&task.kind).to_owned(),
             state: background_state_label(task.state).to_owned(),
             exit_code,
@@ -5814,6 +5822,7 @@ mod tests {
         let conversation_id = ConversationId::from_uuid(Uuid::from_u128(13));
         let state = AgentBackgroundTaskState {
             task_id: TaskId::from_uuid(Uuid::from_u128(14)),
+            parent_task_id: Some(TaskId::from_uuid(Uuid::from_u128(10))),
             kind: BackgroundTaskKindLabel::HeadlessRendezvous,
             state: BackgroundTaskState::Completed,
             exit_code: None,
@@ -5832,5 +5841,9 @@ mod tests {
         assert_eq!(json["requesterAgentId"], requester.to_string());
         assert_eq!(json["targetAgentId"], target.to_string());
         assert_eq!(json["conversationId"], conversation_id.to_string());
+        assert_eq!(
+            json["parentTaskId"],
+            TaskId::from_uuid(Uuid::from_u128(10)).to_string()
+        );
     }
 }