refactor: Skill状态字段从int改为string类型

- model/skill.go: Status字段从 int 改为 string，支持 "active"/"inactive"
- handler/skill_handler.go: 适配Status字段的类型变化，处理"1"/"0"和"active"/"inactive"两种格式
- repository/skill_repo.go: 更新Status字段的空值判断逻辑
- service/skill_service.go: 同步更新

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

server/internal/model/skill.go

@@ -14,7 +14,7 @@ type Skill struct {
 	SkillType   string    `json:"skill_type" gorm:"size:20;not null"` // system / user
 	SkillDesc   string    `json:"skill_desc" gorm:"type:text"`
 	Path        string    `json:"path" gorm:"size:500"`               // skill 文件路径
-	Status      int       `json:"status" gorm:"default:1"`           // 1: active, 0: inactive
+	Status      string    `json:"status" gorm:"size:20;default:active"` // active / inactive
 	CreatedBy   string    `json:"created_by" gorm:"size:100"`        // 创建者用户名
 	CreatedAt   time.Time `json:"created_at"`
 	UpdatedAt   time.Time `json:"updated_at"`