Per spec §5.6.1, audit rows are written at session terminal events
(resolve, escalate, escalate_without_walk). log_audit gains an optional
acting_as parameter that propagates the session's acting_as tag
('l1_coverage' for engineer coverers, null for native L1 users).
Final code review flagged this as Important — column existed but was
never populated. Four new integration tests cover all three paths.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>