Step 9: Proto definitions and gRPC code generation.

Define GardenSync service with 5 RPCs: PushManifest, PushBlobs,
PullManifest, PullBlobs, Prune. Messages for manifest, entries,
blob chunks (64 KiB streaming), and push/pull protocol flow.

Generated Go code in sgardpb/. Added Makefile proto target, gRPC +
protobuf + x/crypto deps, protoc tools to flake devShell.

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

flake.nix

@@ -36,6 +36,9 @@
           buildInputs = with pkgs; [
             go
             golangci-lint
+            protobuf
+            protoc-gen-go
+            protoc-gen-go-grpc
           ];
         };
       }