`terminal(background=true, heartbeat=N)` emits a `heartbeat` event every N seconds
(floor 60) carrying only the output produced since the previous one, plus the usual
completion notice. The agent stays current on a long bounded job (merge train, full
test suite, deploy) and reacts to a failure within N seconds instead of at exit.
Why: `notify=true` fires once at exit, so multi-hour merge trains ran silently — the
parent's prompt cache went cold and every "what's up" cost 5–10 rediscovery calls;
30 days of batch sessions show 4,930 hand status polls (14% of all tool calls).
`notify=[pattern]` cannot serve this: its lifetime cap (8) exists precisely to stop
periodic output from flooding the agent.
Mechanism: one daemon timer thread for every heartbeat session (reader threads block
on the pipe and cannot keep time); delta output via a total-ingested counter over the
rolling buffer; heartbeats only where the completion notice can be delivered (same
async-support/subagent gates), never after exit. `heartbeat_seconds` is checkpointed.
Rendered on CLI, gateway and TUI through the existing process-notification paths.
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