feat(skills): enhance error messages with i18n support

- Add structured error format with error codes and context
- Create skillErrorParser to format errors for user-friendly display
- Add comprehensive i18n keys for all skill-related errors (zh/en)
- Extend download timeout from 15s to 60s to reduce false positives
- Fix: Pass correct error title based on operation context (load/install/uninstall)

Error improvements:
- SKILL_NOT_FOUND: Show skill directory name
- DOWNLOAD_TIMEOUT: Display repo info and timeout duration with network suggestion
- DOWNLOAD_FAILED: Show HTTP status code with specific suggestions (403/404/429)
- SKILL_DIR_NOT_FOUND: Show full path with URL check suggestion
- EMPTY_ARCHIVE: Suggest checking repository URL

Users will now see detailed error messages instead of generic "Error".

src/i18n/locales/zh.json

@@ -675,6 +675,34 @@
     "installFailed": "安装失败",
     "uninstallSuccess": "技能 {{name}} 已卸载",
     "uninstallFailed": "卸载失败",
+    "error": {
+      "skillNotFound": "技能不存在：{{directory}}",
+      "missingRepoInfo": "缺少仓库信息（owner 或 name）",
+      "downloadTimeout": "下载仓库 {{owner}}/{{name}} 超时（{{timeout}}秒）",
+      "downloadTimeoutHint": "请检查网络连接或稍后重试",
+      "skillPathNotFound": "仓库 {{owner}}/{{name}} 中未找到技能路径 '{{path}}'",
+      "skillDirNotFound": "技能目录不存在：{{path}}",
+      "emptyArchive": "下载的压缩包为空",
+      "downloadFailed": "下载失败：HTTP {{status}}",
+      "allBranchesFailed": "所有分支下载失败，尝试了：{{branches}}",
+      "httpError": "HTTP 错误 {{status}}",
+      "http403": "GitHub 访问受限，可能是请求频率过高",
+      "http404": "仓库或分支不存在，请检查地址",
+      "http429": "请求过于频繁，请等待后重试",
+      "parseMetadataFailed": "解析技能元数据失败",
+      "getHomeDirFailed": "无法获取用户主目录",
+      "networkError": "网络错误",
+      "fsError": "文件系统错误",
+      "unknownError": "未知错误",
+      "suggestion": {
+        "checkNetwork": "请检查网络连接",
+        "checkProxy": "建议配置 HTTP 代理",
+        "retryLater": "请稍后重试",
+        "checkRepoUrl": "请检查仓库地址和分支名称",
+        "checkDiskSpace": "请检查磁盘空间",
+        "checkPermission": "请检查目录权限"
+      }
+    },
     "repo": {
       "title": "管理技能仓库",
       "description": "添加或删除 GitHub 技能仓库源",