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/docs.json

@@ -133,6 +133,14 @@
                   "skills/create",
                   "skills/hub"
                 ]
+              },
+              {
+                "group": "内置技能",
+                "pages": [
+                  "skills/skill-creator",
+                  "skills/knowledge-wiki",
+                  "skills/image-generation"
+                ]
               }
             ]
           },
@@ -306,9 +314,16 @@
                 "pages": [
                   "en/skills/index",
                   "en/skills/install",
-                  "en/skills/skill-creator",
                   "en/skills/hub"
                 ]
+              },
+              {
+                "group": "Built-in Skills",
+                "pages": [
+                  "en/skills/skill-creator",
+                  "en/skills/knowledge-wiki",
+                  "en/skills/image-generation"
+                ]
               }
             ]
           },
@@ -485,6 +500,14 @@
                   "ja/skills/create",
                   "ja/skills/hub"
                 ]
+              },
+              {
+                "group": "内蔵スキル",
+                "pages": [
+                  "ja/skills/skill-creator",
+                  "ja/skills/knowledge-wiki",
+                  "ja/skills/image-generation"
+                ]
               }
             ]
           },