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/SideChatView.tsx

@@ -69,6 +69,7 @@ import {
   type SidechatTranscriptRow,
 } from './sidechat-transcript.ts'
 import { api } from './api.ts'
+import { usePolling } from './use-polling.ts'
 import { t } from './locales.ts'
 import type { SessionScope } from './api.ts'
 import type { SidebarTab } from './state.ts'
@@ -493,17 +494,25 @@ export function SideChatView(props: {
     }
   }, [threadId, fetchInfo])
 
-  // Poll while the tab is visible and the thread runs.
+  // One transcript pull on every input change (attach, visibility flip,
+  // run-state flip — the last one catches a thread's terminal state once it
+  // stops running).
   useEffect(() => {
     if (!visible || threadId === undefined) return
     void fetchThread(threadId)
-    if (!running) return
-    const timer = window.setInterval(() => {
-      void fetchThread(threadId)
-      void fetchInfo(threadId)
-    }, POLL_MS)
-    return () => { window.clearInterval(timer) }
-  }, [visible, threadId, running, fetchThread, fetchInfo])
+    // `running` is not read here, but re-triggering this pull on run-state
+    // flips is load-bearing (see above); the badge fetch rides the ticks.
+  }, [visible, threadId, running, fetchThread])
+
+  // Poll while the tab is visible and the thread runs: transcript deltas +
+  // badge refresh on a fixed cadence. Each pull self-guards (fetchThread
+  // aborts its predecessor; a late settle keeps the last rows).
+  const pollTick = useCallback(async (): Promise<void> => {
+    if (threadId === undefined) return
+    void fetchThread(threadId)
+    void fetchInfo(threadId)
+  }, [threadId, fetchThread, fetchInfo])
+  usePolling(visible && running && threadId !== undefined, pollTick, { intervalMs: POLL_MS })
 
   useEffect(() => () => { controllerRef.current?.abort() }, [])