feat(skill): multi-provider image generation with auto-fallback

- Add Gemini, Seedream (Volcengine Ark), Qwen (DashScope), MiniMax
  providers to image-generation skill with universal sequential
  fallback: OpenAI → Gemini → Seedream → Qwen → MiniMax → LinkAI
- Each provider filters unsupported size tiers to valid values
  (e.g. Seedream 1K→2K, Qwen 3K→2K, Gemini 3K→2K)
- Pinned model only tries its native provider; auto-routing uses
  each provider's default model
- Support skill-namespaced config (config.skill.image-generation.model
  → SKILL_IMAGE_GENERATION_MODEL env var)
- Add image lightbox (click-to-enlarge) in web console
- Add  docs for built-in skills (skill-creator, knowledge-wiki,
  image-generation) under docs/skills/

docs/tools/index.mdx

@@ -50,7 +50,7 @@ description: CowAgent 内置工具系统
   <Card title="web_search - 联网搜索" icon="magnifying-glass" href="/tools/web-search">
     搜索互联网获取实时信息
   </Card>
-  <Card title="vision - 图片分析" icon="eye" href="/tools/vision">
+  <Card title="vision - 图片理解" icon="eye" href="/tools/vision">
     分析图片内容（识别、描述、OCR 文字提取等）
   </Card>
   <Card title="browser - 浏览器" icon="window" href="/tools/browser">

docs/tools/vision.mdx

@@ -1,5 +1,5 @@
 ---
-title: vision - 图片分析
+title: vision - 图片理解
 description: 分析图片内容（识别、描述、OCR 等）
 ---