Three start_* functions creating L1WalkSession rows with appropriate
session_kind and target id. Engineers acting in L1 mode get
acting_as='l1_coverage' for audit; native l1_tech users get acting_as=None.
step/notes (T13) and resolve/escalate (T14) extend this file next.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>