codeSplitter threads: run cards take pushed run states, the 1.5 s busy poll goes #3737

Left from #3714 (PR #3736).

#3736 removed the crew file page's idle 10 s poll of GET .../agents: a run that starts or ends now pings its file's socket. While a run is going, the file page (and the repo page) still ask for the run list every 1.5 to 2 s. That poll carries each run card's latest log line and status.

#3731 pushes run states over the thread socket ({t: "run"} from AgentThread). Once it lands, the run cards on the file and repo pages can take those pushes, or the file's runs ping can carry the card, and the busy poll can go.

• Where: pollAgents in client.html (file page), and the repo page's poll next to drawRuns.
• Done when no page polls GET .../agents while a run is going, and a card's status and log line still update live.

Part of #3423.