Align with folk-plugin-grpc 0.2.2 which uses execute_value + base64
instead of msgpack for binary protobuf data.

- GrpcRequest::fromPayload(): base64_decode incoming payload string
- WorkerLoop/EmbedLoop: base64_encode outgoing gRPC response

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>