feat: 新增获取skill内容的API路由

- 新增 GET /skill/content 获取skill的SKILL.md内容

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

server/cmd/api/main.go

@@ -491,6 +491,7 @@ func main() {
 		skillGroup.GET("/list", skillHandler.List)
 		skillGroup.GET("/sync", skillHandler.Sync)
 		skillGroup.GET("/:id", skillHandler.GetByID)
+		skillGroup.GET("/content", skillHandler.GetSkillContent)
 		skillGroup.POST("/add", skillHandler.Create)
 		skillGroup.PUT("/:id", skillHandler.Update)
 		skillGroup.DELETE("/:id", skillHandler.Delete)