fix(skills): resolve third-party skills installation failure (#268)

- Add skills_path field to Skill struct
- Use skills_path to construct correct source path during installation
- Fix installation for repos with custom skill subdirectories

src/lib/api/skills.ts

@@ -10,6 +10,7 @@ export interface Skill {
   repoOwner?: string;
   repoName?: string;
   repoBranch?: string;
+  skillsPath?: string; // 技能所在的子目录路径，如 "skills"
 }
 
 export interface SkillRepo {