feat: 打通 Dispatcher→MCP 工具调用链路 (core NATS request-reply)

第 4 层 Dispatcher 经 NATS request-reply + 队列组同步调用第 5 层 MCP 工具，
工具不可用/超时即降级，不阻断主流程。

- shared/contract: ToolCall/ToolResult + sundynix.tools.go.* subject 约定 + ToolSubjectGo/Py
- shared/bus: CallTool(发起) / ServeTool(队列组订阅+应答)
- mcp-go: 接共享 bus，gateway 通配订阅按工具名分发(wiki_search/echo)，main 优雅退出
- dispatcher: ToolCaller 接口 + Orchestrator.retrieveContext(调 wiki_search,超时3s降级)
- e2e: TestToolCallRoundTrip(PASS)；demo.sh 加 mcp-go(就绪门避免启动竞态)，live 跑通

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-mcp-go/cmd/server/main.go

@@ -2,18 +2,43 @@
 package main
 
 import (
+	"context"
 	"log"
+	"os"
+	"os/signal"
+	"syscall"
+
+	sharedbus "github.com/sundynix/sundynix-shared/bus"
 
 	"github.com/sundynix/sundynix-mcp-go/internal/mcp"
 	"github.com/sundynix/sundynix-mcp-go/internal/search"
 )
 
 func main() {
-	engine := search.NewHybrid() // LLM Wiki 混合检索：Bleve + Milvus + Neo4j
-	gw := mcp.NewGateway(engine)
+	natsURL := envOr("NATS_URL", "nats://localhost:4222")
 
-	log.Println("[mcp_go] serving MCP over sundynix.tools.go.*")
-	if err := gw.Serve(); err != nil {
+	b, err := sharedbus.Connect(natsURL)
+	if err != nil {
+		log.Fatalf("[mcp_go] nats connect: %v", err)
+	}
+	defer b.Close()
+	log.Printf("[mcp_go] connected %s", natsURL)
+
+	engine := search.NewHybrid() // LLM Wiki 混合检索：Bleve + Milvus + Neo4j
+	gw := mcp.NewGateway(b, engine)
+
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	log.Println("[mcp_go] serving MCP over sundynix.tools.go.* (Ctrl-C to quit)")
+	if err := gw.Serve(ctx); err != nil && err != context.Canceled {
 		log.Fatalf("[mcp_go] exit: %v", err)
 	}
 }
+
+func envOr(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}