Build bin/storage-migrate alongside the server binary

Why: the U4 managed migration entry must ship with make build so operators and CI get the tool.

Evidence: make build emits both binaries; migrator suite green on SQLite and real PostgreSQL.

Impact: build target output adds bin/storage-migrate; no test-path change.

Refs: backend PR #144; runbook section 10.

Makefile

@@ -93,7 +93,8 @@ frontend: contracts
 
 build: frontend
 	cd $(BACKEND) && CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.version=$(VERSION)" -o ../../$(BIN) ./cmd/agent-system
-	@echo "OK: $(BIN)（版本 $(VERSION)；随附插件目录 $(PLUGINS)/）"
+	cd $(BACKEND) && CGO_ENABLED=0 go build -trimpath -ldflags "-s -w" -o ../../bin/storage-migrate ./cmd/storage-migrate
+	@echo "OK: $(BIN)（版本 $(VERSION)；随附插件目录 $(PLUGINS)/）+ bin/storage-migrate（协作存储 U4 迁移工具）"
 
 test-core:
 	cd $(CORE) && go test ./...