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>

Makefile

@@ -0,0 +1,20 @@
+.PHONY: proto build test lint clean
+
+proto:
+	protoc \
+		--go_out=. --go_opt=module=github.com/kisom/sgard \
+		--go-grpc_out=. --go-grpc_opt=module=github.com/kisom/sgard \
+		-I proto \
+		proto/sgard/v1/sgard.proto
+
+build:
+	go build ./...
+
+test:
+	go test ./...
+
+lint:
+	golangci-lint run ./...
+
+clean:
+	rm -f sgard