refactor: 清理废弃字段，同步 Schema 定义

- Agent 文件：删除 downloads、rating、ratingCount、installStatus、source
- Skill 文件：删除 downloads、rating、ratingCount、installStatus、source、version
- Skill 文件：新增 files 字段，指向配置文件路径
- 字段说明：source 和 installStatus 改为运行时计算

Refs: desirecore/desirecore#field-recovery

agents/writing-coach/agent.json

@@ -8,7 +8,13 @@
   "shortDesc": "从构思到润色，全方位提升你的写作能力",
   "fullDesc": "写作教练是你专属的文案合伙人。无论你是写技术博客、产品文档还是营销文案，它都能帮你找到合适的表达方式——清晰、有说服力、有你的个人风格。\n\n服务范畴涵盖：\n- 内容策划：选题挖掘、结构规划、受众定位\n- 风格诊断：语气一致性、品牌调性匹配、文化敏感度\n- 语言润色：消除冗余、增强节奏感、优化修辞\n- 格式规范：Markdown 排版、文档结构、视觉层次\n- 多语言适配：中英文写作差异、本地化建议",
   "category": "creative",
-  "tags": ["写作", "内容创作", "文案", "技术写作", "创意写作"],
+  "tags": [
+    "写作",
+    "内容创作",
+    "文案",
+    "技术写作",
+    "创意写作"
+  ],
   "version": "2.0.5",
   "latestVersion": "2.0.5",
   "updatedAt": "2025-02-14",
@@ -16,14 +22,21 @@
     "name": "DesireCore Official",
     "verified": true
   },
-  "downloads": 25680,
-  "rating": 4.9,
-  "ratingCount": 723,
-  "installStatus": "not_installed",
   "persona": {
     "role": "资深编辑与写作导师",
-    "traits": ["语感敏锐", "耐心细致", "尊重作者风格", "善于激发创意"],
-    "tools": ["style_analyzer", "readability_checker", "plagiarism_detector", "thesaurus_api", "format_linter"]
+    "traits": [
+      "语感敏锐",
+      "耐心细致",
+      "尊重作者风格",
+      "善于激发创意"
+    ],
+    "tools": [
+      "style_analyzer",
+      "readability_checker",
+      "plagiarism_detector",
+      "thesaurus_api",
+      "format_linter"
+    ]
   },
   "changelog": "v2.0.5: 新增小说创作模式，支持长文一致性检查"
 }