DOCFIX-131: tenant-offboard.sh Phase A cluster captures stdout-only ...

Fixes the last merged-stderr sites in the offboard (Phase C/D were DOCFIX-113).
A benign 'coe cluster list' stderr warning could word-split into the delete loop
and keep the wait loop from ever seeing empty -- a false 'not gone' exit 21 on an
already-clean tenant. New tenant-scoped tinv() (stdout-only, stderr surfaced as
NOTE); wait loop retries on list-error, final check fails closed. Harness un-excludes
coe from warnstderr + new coe-stderr-not-argv assertion (22/22). Gauntlet 38/38.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>