perf(renderer): stop closed workspace modals scanning and sorting every agent #782

React-wide audit found AgentActivityModal, CloseOldAgentsModal and BulkProviderSwitchModal remain mounted behind closed Dialogs while their useMemo row builders still scan tabs/sessions on every workspace runtime replacement. Close Old Agents additionally scans retained transcript entries for last activity, then groups/sorts rows and projects. Their timers are open-gated but the expensive derived rows are not (AgentActivity even describes them as guarded by open). Expected: no session enumeration/row derivation while closed; reopening derives current state immediately; visible previews stay live; close/switch actions retain live revalidation. Keep mounted component state and existing effects rather than changing modal lifetimes. Regression: deliver repeated runtime replacements to a closed modal and assert no enumeration, then open/update/close and verify derivation happens only while visible. Refs #763, #103.