fix(harness): preserve session migration JSON errors

.superpowers/sdd/task-2-report.md

@@ -149,3 +149,29 @@ session while the adapter executes as `thoth_sessions_runtime`.
 - Task 7 must safely provision the dedicated runtime login/membership and inject its TLS
   credentials/CA; this task intentionally does not create a deployment-specific LOGIN role or
   password.
+
+## Review follow-up — unavailable migration database JSON contract
+
+### RED
+
+Command:
+
+```sh
+cd harness && .venv/bin/pytest \
+  tests/test_session_migrate_cmd.py::test_session_migrate_status_database_failure_is_pristine_json -q
+```
+
+Result: `1 failed in 0.46s`. The unreachable direct PostgreSQL URL exited with code 1 but left
+stdout empty, so `json.loads(result.stdout)` raised `JSONDecodeError`.
+
+### GREEN
+
+The session migration CLI now catches `SQLAlchemyError` at the same command boundary as its
+migration/domain errors and emits only `{"error": ...}` on stdout for `--json`.
+
+```sh
+cd harness && .venv/bin/pytest tests/test_session_migrate_cmd.py -q
+cd harness && .venv/bin/ruff check tht/cli/session_cmd.py tests/test_session_migrate_cmd.py
+```
+
+Result: `2 passed in 3.60s`; Ruff: `All checks passed!`.