Summary

Skip requestHeartbeatNow for subagent session keys in both maybeNotifyOnExit() and emitExecSystemEvent(). Subagent sessions receive exec results via process poll and report completion via subagent_announce, so the exec-event heartbeat wake is redundant and wasteful.

Problem

Each background exec completion in a subagent triggers requestHeartbeatNow({reason: "exec-event"}). Since resolveHeartbeatSession falls back subagent keys to the main session, this causes spurious heartbeat wake-ups (and LLM calls) on the parent agent every time a subagent exec finishes.

In a session with 47 subagent exec calls, this produced ~50 unnecessary LLM invocations.

Changes

• Guard both requestHeartbeatNow call sites with isSubagentSessionKey(sessionKey)
• Add import for isSubagentSessionKey from session-key-utils.js
• Add test case verifying heartbeat wake is skipped for subagent session keys

Testing

• New test: emitExecSystemEvent with subagent session key → requestHeartbeatNow not called (but enqueueSystemEvent still fires)
• Existing tests for non-subagent and empty session keys remain unchanged

Fixes openclaw#66748