refactor(client): 四处轮询站点迁移到 use-polling，语义逐一保持

- SubagentView useSubagentLive：self-scheduling（上一请求 settle 后才排
  下一次，不重叠）+ 每代 AbortSignal + signal.aborted 拦迟到写入，行为
  与原 disposed 标志等价（teardown 同步置 disposed 并 abort）。
- SideChatView：立即拉取 effect 与 2s 定时拆分——输入变化（含 running
  翻转）仍先拉一次转录，运行中间隔 tick 拉转录+徽章；fetchThread 自带
  的逐次 abort 与卸载 abort 原样保留。
- GitLens：可见时 2s 固定静默 tick，无初始突发（mount/scope 变更的完整
  刷新在别处）；refresh 身份变化照旧重启间隔相位。
- ChangesTab：输入变化即拉一次（含隐藏态翻转），可见时 2.5s 固定间隔；
  pull 内部 pollGen 代际守卫不动。

守护 spec 全绿：subagent-live-polling + subagent-jobs-view 13/13、
sidechat-view + sidechat-transcript 29/29、git-view-worktree +
changes-tab + changes-ops 13/13；断言零改动。

src/client/changes/ChangesTab.tsx

@@ -18,6 +18,7 @@ import type { SidebarSessionEvent } from '../../context-types.ts'
 import type { TabComponentProps } from '../service.ts'
 import { t } from '../locales.ts'
 import { api } from '../api.ts'
+import { usePolling } from '../use-polling.ts'
 import { floatTab, type SidebarDiffRef } from '../state.ts'
 import { GitLens } from './GitLens.tsx'
 import { SessionLens } from './SessionLens.tsx'
@@ -104,12 +105,11 @@ export function ChangesTab({ ctx, store, scope, tab, visible, onOpenFile, onOpen
     seqRef.current = 0
     setOpsError(false)
   }, [scope.sessionId])
-  useEffect(() => {
-    void pull()
-    if (!visible) return
-    const timer = window.setInterval(() => { void pull() }, 2_500)
-    return () => { window.clearInterval(timer) }
-  }, [visible, pull])
+  // One pull on every input change — mount, scope change, and visibility
+  // flip all re-pull (a hidden tab still catches up on the flip); only the
+  // poll CADENCE below is gated by visibility.
+  useEffect(() => { void pull() }, [visible, pull])
+  usePolling(visible, pull, { intervalMs: 2_500 })
   // tick only forces the re-render; the fold reads the ref directly.
   void tick
   const ops = opsRef.current