feat(skills): 新增 dashscope-image-gen 和 xiaomi-tts 全局内置技能

- dashscope-image-gen: 阿里云通义万相文生图技能（wan2.7-image 系列）
- xiaomi-tts: 小米 MiMo 语音合成技能（mimo-v2.5-tts 系列）
- 更新 builtin-skills.json 清单

builtin-skills.json

@@ -1,6 +1,7 @@
 {
   "skills": [
     "create-agent",
+    "dashscope-image-gen",
     "delete-agent",
     "dev-environment-setup",
     "discover-agent",
@@ -21,6 +22,7 @@
     "update-agent",
     "web-access",
     "workflow",
+    "xiaomi-tts",
     "xlsx"
   ]
 }