feat(delegate-task): ⚙️ make sync subagent timeout configurable via syncPollTimeoutMs

Allow users to set `background_task.syncPollTimeoutMs` in config to override
the default 10-minute sync subagent timeout. Affects sync task, sync continuation,
and unstable agent task paths. Minimum value: 60000ms (1 minute).

Co-authored-by: Wine Fox <fox@ling.plus>

src/config/schema/background-task.ts

@@ -8,6 +8,7 @@ export const BackgroundTaskConfigSchema = z.object({
   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(),
+  syncPollTimeoutMs: z.number().min(60000).optional(),
 })
 
 export type BackgroundTaskConfig = z.infer<typeof BackgroundTaskConfigSchema>