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/service/skill_service.go

@@ -143,7 +143,7 @@ func (s *SkillService) scanSkillsDirectory(basePath string, skillType string) ([
 			SkillType: skillType,
 			SkillDesc: skillInfo.SkillDesc,
 			Path:      skillPath,
-			Status:    1,
+			Status:    "active",
 		}
 
 		skills = append(skills, skill)