Remove zod-to-json-schema from packages/shared (replaced by Zod v4 built-in)

packages/shared depends on zod-to-json-schema@3.25.2 but it doesn't work with Zod v4 (produces empty schemas). The storage-foundation plan switched to Zod v4's built-in `toJSONSchema` instead. The zod-to-json-schema package should be uninstalled from packages/shared to clean up the unused dependency.