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/tools/delegate-task/sync-session-poller.test.ts

@@ -273,7 +273,7 @@ describe("pollSyncSession", () => {
         agentToUse: "test-agent",
         toastManager: null,
         taskId: undefined,
-      })
+      }, 0)
 
       //#then - timeout returns error string
       expect(result).toBe("Poll timeout reached after 50ms for session ses_timeout")