perf(workbench): pause task polling while browsing history

[Qiyuanqiii]

Summary

Loading task history made every 15-second sidebar poll refetch all cached pages sequentially. Poll only while the visible conversation sidebar has a single cached page. Multi-page history retains manual refresh, existing invalidations, focus and reconnect refreshes; switching to a single-page search resumes polling. Historical rows retain their last observed status until refreshed. Usage guidance and ADR 0167 document that behavior.

Follow-up to #272 / #283. No task pages, selected task or composer draft are discarded.

Validation

• Three new regressions fail on the original implementation and pass after the change. With five cached pages, 45 seconds previously added 15 list requests; the periodic request count is now zero, for both unfiltered and title-filtered history.
• Tests verify manual refresh and invalidation update every cached page, single-page search resumes status polling, restoring cached history pauses polling again, and task/draft state is preserved.
• Task search and sidebar suites: 35 tests pass. Workbench integration suite: 34 tests pass.
• TypeScript typecheck, production build, npm run check:api, and git diff --check pass.
• CI run 37594541711 passes on commit 14211e1f33db736209e58c4af1c875d68b5885d6, based on main 6509627e59e19b17ee61f4f964199dec962f784a: all four applicable checks pass, with nine conditional checks skipped. Complete frontend suite: 174 files / 1,642 tests pass; production build, generated API check and dependency audit (zero vulnerabilities) pass. Go documentation/release contracts and the control-plane aggregate pass.
• Go/CLI runtime changes: N/A. Existing applicable CI is retained without adding a workflow or duplicate gate.

Surface governance

• No Surface is added, promoted, downgraded, deprecated, or removed.
• Registry declarations: N/A — no Surface change.

Audit

• No credentials or local runtime data are included.
• This change adjusts only frontend periodic reads; authorization, workspace scope and persistence are unchanged.
• Usage guidance and the existing task-search ADR describe polling and historical-state freshness.