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/domain/index.ts

@@ -612,7 +612,15 @@ export interface BackgroundCompletion {
   ownerAgentId: string;
   projectId: string;
   kind: string;
-  status: "running" | "completed" | "failed" | "cancelled" | "pending" | "delivered";
+  status:
+    | "running"
+    | "cancelling"
+    | "completed"
+    | "failed"
+    | "cancelFailed"
+    | "cancelled"
+    | "pending"
+    | "delivered";
   exitCode: number | null;
   /** Human-readable summary / error / reason of a terminal result (ticket #5). */
   summary: string | null;