Reflection follow-up: investigate why reflection-filed tasks 1ea141a9 + 9f168f38 stalled in queue >24h; identify heartbeat-pickup gate

From 2026-06-22 reflection: fleet-tasks 1ea141a9-e32c-4cfa-918f-410c60b94291 (canonical-store pre-check) and 9f168f38-1ef3-4b46-bff9-4d00f3bb43b1 (rebase-stale-prs.sh) were both filed at 04:11-04:12 UTC on 2026-06-21. Direct fleet-tasks API check at 04:05 UTC 2026-06-22 confirms both still status=queued with ONLY a single created event each — no claimed, no in_progress, no completion event. This is the first stall of the reflection→action loop after 8 consecutive on-budget datapoints across 8 nights.\n\nHypotheses to investigate (declining order of fit):\n1. Sunday-quiet cascade hit the agent side: Will has not merged any PR since 2026-06-18 14:37-16:11 UTC batch (~4 days). If the heartbeat-pickup logic gates on workspace merge events to decide queue-draining, no merges = no warmth = no pickup. The loops prior 8 successes may have all happened on days when Will was actively merging, which kept the heartbeat warm.\n2. Queue FIFO within priority pushed the new priority-3 tasks behind 15 older priority-3 Wave-2/-3 tasks that are structurally unstarted because they are blocked on PR-merges-that-have-not-happened. New tasks landed at back of queue.\n3. Heartbeat cron drift / error halted pickup entirely.\n\nAcceptance: identify the actual root cause via heartbeat code path inspection or queue-drain log analysis. Document in next reflection. If the root cause is hypothesis 1, the loop is fundamentally event-driven and needs a time-triggered fallback (e.g. once-per-24h queue-drain regardless of warmth). If hypothesis 2, the priority sort within FIFO should be re-examined. If hypothesis 3, fix the cron/heartbeat.\n\nThis is priority 2 because it investigates a regression in a primitive (the reflection→action loop) that was promoted to substrate-level in last nights reflection. A stalled primitive that the framework relies on for self-correction is a higher-priority investigation than feature work.