fix(runtime): cancel de background task immédiat et fiable depuis le workstate (#199, QA verte)

Transforme l'annulation d'une background task en contrat immédiat, observable et non-best-effort:

- domain/background_task: états Cancelling/Cancelled/CancelFailed et transitions idempotentes; un kill PTY qui échoue est surfacé en CancelFailed plutôt qu'en succès silencieux.

- infrastructure/runner + store: cancel effectif de la task ciblée, reconcile marque Cancelling même sans live handle, et départage CancelFailed des tâches pending au boot.

- application/background + workstate + orchestrator/wake: chemin de cancel initié depuis le workstate, projection synchronisée de l'état (pas d'attente d'un cycle ambigu).

- frontend: projection workstate de l'état cancelled/cancelling/cancelFailed (cancellingRow/cancelFailedRow/cancelButtons), normalization et services.

Tests QA verts: background_task_cancel, background_task_runner cancel_wins_even_if_output_is_still_available, background_task_store reconcile_marks_cancelling_without_live_handle_cancel_failed_and_pending, workstate.test.tsx.

crates/application/src/orchestrator/service.rs

@@ -942,12 +942,14 @@ impl OrchestratorService {
                 owner_agent_id,
                 rendezvous,
             },
-            BackgroundTaskResult::Failure { .. } => DomainEvent::BackgroundTaskFailed {
-                project_id: project.id,
-                task_id,
-                owner_agent_id,
-                rendezvous,
-            },
+            BackgroundTaskResult::Failure { .. } | BackgroundTaskResult::CancelFailed { .. } => {
+                DomainEvent::BackgroundTaskFailed {
+                    project_id: project.id,
+                    task_id,
+                    owner_agent_id,
+                    rendezvous,
+                }
+            }
             BackgroundTaskResult::Cancelled { .. } | BackgroundTaskResult::Expired { .. } => {
                 DomainEvent::BackgroundTaskCancelled {
                     project_id: project.id,
@@ -1333,9 +1335,11 @@ impl OrchestratorService {
             BackgroundTaskState::Queued => "Queued",
             BackgroundTaskState::Running => "Running",
             BackgroundTaskState::Waiting => "Waiting",
+            BackgroundTaskState::Cancelling => "Cancelling",
             BackgroundTaskState::Completed => "Completed",
             BackgroundTaskState::Failed => "Failed",
             BackgroundTaskState::Cancelled => "Cancelled",
+            BackgroundTaskState::CancelFailed => "CancelFailed",
             BackgroundTaskState::Expired => "Expired",
         };
         Ok(OrchestratorOutcome {

crates/application/src/orchestrator/wake.rs

@@ -317,6 +317,10 @@ fn background_completion_prompt(task: &BackgroundTask) -> String {
             "La tâche de fond {} est terminée : annulée.\nRaison : {}",
             task.id, reason
         ),
+        Some(BackgroundTaskResult::CancelFailed { error, .. }) => format!(
+            "La tâche de fond {} est terminée : échec de l'annulation.\nErreur : {}",
+            task.id, error
+        ),
         Some(BackgroundTaskResult::Expired { reason, .. }) => format!(
             "La tâche de fond {} est terminée : expirée.\nRaison : {}",
             task.id, reason