feat: manage_skill + 全局 skills 目录

新增:
- manage_skill: LLM 可调用的 Skill 创建/删除/列出工具 (High)
- 全局 skills 支持: ~/.ias/skills/ 自动加载
- SkillRegistry::register_skill() 动态注册
- SkillRegistry::load() 三元加载: system → global → user

manage_skill 操作:
  create: 写 SKILL.md + 执行脚本到 skills/<name>/
  delete: 移除 skills/<name>/
  list: 列出所有自定义 Skill

目前一共 9 个系统 Skills

resources/skills/manage_skill/SKILL.md

@@ -0,0 +1,49 @@
+# manage_skill
+
+管理 Skill：创建新的 Skill，列出所有已加载的 Skill，删除 Skill。
+
+创建的 Skill 会持久化到 `skills/` 目录，重启后自动加载。
+
+## Risk Level
+High
+
+## Parameters
+```json
+{
+  "type": "object",
+  "properties": {
+    "action": {
+      "type": "string",
+      "description": "操作类型",
+      "enum": ["create", "delete", "list"]
+    },
+    "name": {
+      "type": "string",
+      "description": "Skill 名称（create/delete 时需要）"
+    },
+    "description": {
+      "type": "string",
+      "description": "Skill 描述（create 时需要）"
+    },
+    "command": {
+      "type": "string",
+      "description": "Skill 执行的命令（create 时需要）"
+    },
+    "parameters": {
+      "type": "string",
+      "description": "参数 JSON Schema（create 时需要，JSON 字符串）"
+    },
+    "risk_level": {
+      "type": "string",
+      "description": "风险等级 Low|High，默认 Low",
+      "enum": ["Low", "High"]
+    }
+  },
+  "required": ["action"]
+}
+```
+
+## Execute
+```bash
+scripts/manage.sh
+```