fix(background-agent): detect stale tasks that never received progress updates

Tasks with no progress.lastUpdate were silently skipped in
checkAndInterruptStaleTasks, causing them to hang forever when the model
hangs before its first tool call. Now falls back to checking startedAt
against a configurable messageStalenessTimeoutMs (default: 10 minutes).

Closes #1769

src/config/schema/background-task.ts

@@ -6,6 +6,8 @@ export const BackgroundTaskConfigSchema = z.object({
   modelConcurrency: z.record(z.string(), z.number().min(0)).optional(),
   /** Stale timeout in milliseconds - interrupt tasks with no activity for this duration (default: 180000 = 3 minutes, minimum: 60000 = 1 minute) */
   staleTimeoutMs: z.number().min(60000).optional(),
+  /** Timeout for tasks that never received any progress update, falling back to startedAt (default: 600000 = 10 minutes, minimum: 60000 = 1 minute) */
+  messageStalenessTimeoutMs: z.number().min(60000).optional(),
 })
 
 export type BackgroundTaskConfig = z.infer<typeof BackgroundTaskConfigSchema>