feat: 更新后端agent和skill服务

- agent_handler.go: 新增ListAgents、CreateAgent接口
- skill_handler.go: 更新skill内容获取和保存功能
- agent_service.go: 新增agent服务逻辑
- main.go: 新增agent路由

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/cmd/api/main.go

@@ -500,6 +500,8 @@ func main() {
 	// Agent管理模块 (调用 Python Agent Engine)
 	agentGroup := r.Group("/api/agent")
 	{
+		agentGroup.GET("/list", agentHandler.ListAgents)
+		agentGroup.POST("/create", agentHandler.CreateAgent)
 		agentGroup.POST("/chat", agentHandler.Chat)
 		agentGroup.POST("/chat/stream", agentHandler.ChatStream)
 		agentGroup.POST("/team/chat", agentHandler.TeamChat)