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

scripts/seed-development-demo.sh

@@ -78,8 +78,12 @@ migration_head="$(
     "${compose[@]}" exec -T postgres sh -ec \
         'psql -v ON_ERROR_STOP=1 -At -U "$POSTGRES_USER" -d "$POSTGRES_DB" -c "SELECT version_num FROM alembic_version"'
 )"
-if [[ "${migration_head}" != "0016_agent_zones" ]]; then
-    echo "Refusing demo seed: database Alembic head is ${migration_head:-missing}, expected 0016_agent_zones." >&2
+expected_head="$(
+    "${compose[@]}" run --rm --no-deps backend alembic heads 2>/dev/null |
+        awk 'NR == 1 { print $1 }'
+)"
+if [[ -z "${expected_head}" || "${migration_head}" != "${expected_head}" ]]; then
+    echo "Refusing demo seed: database Alembic head is ${migration_head:-missing}, expected ${expected_head:-unresolved}." >&2
     exit 1
 fi