feat(app): add composition root, stageflow, ContextUpdate, and SSE transport

Move construction and document OnOutbox out of server, give HTTP/CLI a shared task use case, and block already-cleared import/event arch rules.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/server/main.go

@@ -10,7 +10,7 @@ import (
 	"syscall"
 	"time"
 
-	"chat-agent-system/internal/bizdev"
+	"chat-agent-system/internal/app"
 	"chat-agent-system/internal/config"
 	"chat-agent-system/internal/server"
 )
@@ -38,12 +38,16 @@ func main() {
 		cfg.AgentBackend = *backend
 	}
 
-	svc, err := bizdev.NewService(cfg)
+	rt, err := app.New(cfg)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "业务服务初始化失败: %v\n", err)
 		os.Exit(1)
 	}
-	srv := server.NewServer(cfg, svc)
+	if err := rt.Start(context.Background()); err != nil {
+		fmt.Fprintf(os.Stderr, "后台 worker 启动失败: %v\n", err)
+		os.Exit(1)
+	}
+	srv := server.NewFromRuntime(rt)
 
 	addr := fmt.Sprintf("%s:%d", cfg.HTTPHost, cfg.HTTPPort)
 	fmt.Println("==================================================================")