Audit: sub-agent completion content vs heartbeat-steer content (the 'poll.' confusable-input)

Today task f12c0494 (BOLT-1268 P5 SQL rework) had its sub-agent 5df60bc6 complete at 21:40 UTC with the literal result string 'completed with non-substantive reply "poll." — investigating actual work in worktree before deciding next step'. Work was eventually recovered as commit de0855e6b6. Hypothesis: the sub-agent received a heartbeat-side `poll` steer mid-run and round-tripped it as its completion message. Sub-agent confused inbox for outbox. Investigation steps: 1. Find the sub-agent transcript for session 5df60bc6. 2. Identify whether a `poll` steer was sent during its run. 3. Check the sub-agent spawn prompt and completion contract — is there ambiguity between an inbound steer and an outbound completion ack? 4. If yes: namespace heartbeat steers with prefix HEARTBEAT_POLL: or a structured {action:'poll'} payload. If this is ambient (could explain other no-op completions), grep memory/ for similar single-word completion strings. First enforcement of the new rule (LEARNINGS 2026-06-13): sub-agent completion content must be namespaced from heartbeat-steer content.