Problem

When a subagent runs background exec commands (e.g. background: true), each exec completion triggers requestHeartbeatNow({reason: "exec-event"}). The heartbeat runner's resolveHeartbeatSession falls back subagent session keys to the main session, causing spurious heartbeat wake-ups on the parent agent.

This results in:

• Frequent unnecessary LLM calls (one per subagent exec completion)
• Spurious HEARTBEAT_OK messages appearing in the main session (visible in WebChat)
• Wasted API tokens

Reproduction

1. Open WebChat for the main agent
2. Spawn a subagent that runs multiple background exec commands:

   sessions_spawn(task: "...", mode: "run")
   

3. Observe the main session receiving HEARTBEAT_OK responses every 60-90 seconds

Root Cause

In src/agents/bash-tools.exec-runtime.ts, both maybeNotifyOnExit() and emitExecSystemEvent() unconditionally call:

requestHeartbeatNow(scopedHeartbeatWakeOptions(sessionKey, { reason: "exec-event" }));

In src/infra/heartbeat-runner.ts, resolveHeartbeatSession() (line 253) falls back subagent session keys:

if (!trimmed || isSubagentSessionKey(trimmed)) {
    return { sessionKey: mainSessionKey, ... };
}

This is redundant because:

• Subagents receive exec results via normal process poll — they are actively waiting
• Subagent completion is communicated to the parent via subagent_announce
• The heartbeat provides no value for subagent sessions

Proposed Fix

Guard the requestHeartbeatNow calls with isSubagentSessionKey:

if (!isSubagentSessionKey(sessionKey)) {
  requestHeartbeatNow(scopedHeartbeatWakeOptions(sessionKey, { reason: "exec-event" }));
}

PR incoming.