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.

frontend/src/features/workstate/ProjectWorkStatePanel.tsx

@@ -92,10 +92,14 @@ function formatTaskStatus(status: BackgroundCompletion["status"]): string {
   switch (status) {
     case "running":
       return "Running";
+    case "cancelling":
+      return "Cancelling";
     case "completed":
       return "Completed";
     case "failed":
       return "Failed";
+    case "cancelFailed":
+      return "Cancel failed";
     case "cancelled":
       return "Cancelled";
     case "delivered":
@@ -107,16 +111,33 @@ function formatTaskStatus(status: BackgroundCompletion["status"]): string {
 
 /** Terminal (finished) task states that carry a result summary (ticket #5). */
 function isTerminalTaskStatus(status: BackgroundCompletion["status"]): boolean {
-  return status === "completed" || status === "failed" || status === "cancelled";
+  return (
+    status === "completed" ||
+    status === "failed" ||
+    status === "cancelFailed" ||
+    status === "cancelled"
+  );
 }
 
 function taskStatusClass(status: BackgroundCompletion["status"]): string {
-  if (status === "running" || status === "pending") return "bg-warning/15 text-warning";
+  if (status === "running" || status === "pending" || status === "cancelling") {
+    return "bg-warning/15 text-warning";
+  }
   if (status === "completed" || status === "delivered") return "bg-success/10 text-success";
-  if (status === "failed" || status === "cancelled") return "bg-danger/10 text-danger";
+  if (status === "failed" || status === "cancelFailed" || status === "cancelled") {
+    return "bg-danger/10 text-danger";
+  }
   return "bg-raised text-muted";
 }
 
+function canCancelTask(status: BackgroundCompletion["status"]): boolean {
+  return status === "running" || status === "pending" || status === "cancelling";
+}
+
+function canRetryTask(status: BackgroundCompletion["status"]): boolean {
+  return status === "failed" || status === "cancelFailed" || status === "cancelled";
+}
+
 function requesterLabel(ticket: AgentTicketState): string {
   if (ticket.source.kind === "human") return "Human";
   const id = shortTicket(ticket.source.agentId);
@@ -346,8 +367,8 @@ function BackgroundTaskRow({
   const attachmentRef = useRef<BackgroundTaskAttachment | null>(null);
   const attachSeqRef = useRef(0);
   const hasOutput = Boolean(task.stdoutTail || task.stderrTail);
-  const canCancel = task.status === "running" || task.status === "pending";
-  const canRetry = task.status === "failed" || task.status === "cancelled";
+  const canCancel = canCancelTask(task.status);
+  const canRetry = canRetryTask(task.status);
   const canAttachLive = canCancel;
 
   function detachLive(): void {