Parent: #64227

Summary

Harden replay and long-task liveness so GPT-5.4 does not silently disappear mid-task or have replay failures misread as model behavior.

Scope

• replay-invalid classification
• stale continuation / compaction abandonment handling
• liveness states for working, paused, blocked, and abandoned
• hardening only around current replay/liveness seams; no duplication of feat: context-pressure-aware continuation (CONTINUE_WORK / CONTINUE_DELEGATE) #38780

Acceptance

• replay failures classify as replay failures
• long-running tasks cannot silently vanish without a user-visible state transition
• compaction/replay transitions preserve truthful progress state