feat(agent): add durable existing-agent credential transitions

scripts/test-miner-control-migration.sh

@@ -130,8 +130,9 @@ run_alembic upgrade 0015_miner_control_foundation
 echo "Testing empty database through head..."
 "${compose[@]}" exec -T postgres sh -ec 'createdb -U "$POSTGRES_USER" minerops_control_empty'
 "${compose[@]}" run --rm --no-deps -e POSTGRES_DB=minerops_control_empty backend alembic upgrade head
+expected_head="$(run_alembic heads 2>/dev/null | awk 'NR == 1 { print $1 }')"
 empty_revision="$("${compose[@]}" exec -T postgres sh -ec \
     'psql -U "$POSTGRES_USER" -d minerops_control_empty -Atc "SELECT version_num FROM alembic_version"')"
-[[ "${empty_revision}" == "0015_miner_control_foundation" ]]
+[[ -n "${expected_head}" && "${empty_revision}" == "${expected_head}" ]]
 
 echo "Miner Control Foundation migration tests passed."