refactor: 删除外置 skills 系统，全部工具转为内置

删除:
- resources/skills/ (9个外置 skill, bash/python/node 脚本)
- src/tools/registry.rs (SkillRegistry 加载/执行)
- src/tools/parser.rs (SKILL.md 解析)
- src/skills_importer.rs (skills 导入)
- cli.rs Skills 子命令
- main.rs cmd_skills / global_skills_dir

保留:
- src/tools/builtin.rs (内置工具注册 + 审批流程)
- src/tools/weather.rs (天气查询)
- src/tools/skill.rs (精简为 SkillSpec/RiskLevel/SkillResult/ExecutionContext)
- src/tools/approval.rs (审批管理器)

main.rs 简化: 移除 SkillRegistry 加载, executor 只走 BuiltinRegistry

Cargo.toml

@@ -7,7 +7,7 @@ edition = "2024"
 # 异步运行时
 tokio = { version = "1.0", features = ["full"] }
 # HTTP 客户端
-reqwest = { version = "0.12", features = ["json", "stream"] }
+reqwest = { version = "0.12", features = ["json", "stream", "gzip"] }
 # 序列化
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"