Skip the 15s heartbeat on parked or settled folds

[Thytu]

Summary

• skip the 15s session liveness heartbeat when the run is parked (idle_since) or the current fold has completed output and nothing in flight
• keep the heartbeat for unresolved tools and unanswered questions
• treat the live-plan chip as chrome, not in-flight work

I reviewed the full diff; this stops a finished turn from looking Working forever just because the heartbeat kept updated_at fresh.

Why this approach

The heartbeat exists so a silent-but-alive child stays inside the UI's 45s Working window. That is right for an open tool or a parked question. It is wrong once the fold already has completed text and no open work: each tick resets freshness, so the 45s staleness gate never fires.

Tradeoff

Thinking that still emits deltas stays fresh through the event path. A silent think with no open tool and no question will now age out of Working after 45s even though the child is still alive. That is the intended cost of letting the dropped-reply / mid-run hang shape settle.

Testing

• cargo test -p zeron-engine fold_liveness
  • completed text / empty fold → no heartbeat
  • open tool + unanswered question → heartbeat
  • live-plan chip alone → no heartbeat

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