Persists this run's entire stdout/stderr transcript to BACKUP_DIR
(/volume1/SMB-docker/backup), the SAME location as the pg_dumpall
backup file itself, so a failed run can be investigated later even
without a live terminal attached. Appends to an already-open
CUTOVER_SESSION_LOG if invoked as a child of cutover.sh (one merged
multi-phase transcript per session); opens its own
preflight-standalone-<TS>.log if run directly. Also logs the repo's
git HEAD at cutover/ for traceability, per the ADR-0001 note's
local-checkout-drift lesson (Session Update 8).
See ADR-0001 note, Session Update 9.