Heartbeat Phase 3.5: PATCH-back-to-in-progress on subagent_completed events

Mirror of awaiting-review protocol (PATCH to blocked when CI green). Inverse mitigation for substrate noise: PATCH back to in_progress when subagent_completed event fires, even if lease has already expired. Reduces poison-pill terminal events that today fired 4x. Logic in bin/lease-renew.sh: 1. Query GET /api/v1/tasks?status=failed AND status=queued (recently) 2. For each, GET /api/v1/tasks/{id}/events?type=subagent_completed within last 30 min 3. If subagent completed AND task is failed (poison-pill) or queued (re-queued from lease expiry), PATCH to in_progress with renewed lease 4. Distinguish from genuine sub-agent failures by checking the completion message has actual substantive content This is Phase 3.5 — not as fundamental as Phase 4 (spawn-time PATCH) but a useful mitigation in case Phase 4 is delayed. Acceptance: poison-pill count on a busy day drops from 4 to 0 even without Phase 4. Reference: LEARNINGS 2026-06-13.