refactor(cli): 重构清屏和which命令实现

1. 提取清屏、设置环境变量、命令查找逻辑到system任务模块
2. 统一命令行工具的任务实现方式，减少重复代码
3. 修正pyproject.toml中的cli命令名拼写错误
4. 移除过时的测试用例代码

pyproject.toml

@@ -25,7 +25,7 @@ version = "0.2.5"
 [project.scripts]
 autofmt     = "pyflowx.cli.autofmt:main"
 bumpversion = "pyflowx.cli.bumpversion:main"
-cls         = "pyflowx.cli.clearscreen:main"
+clr         = "pyflowx.cli.clearscreen:main"
 emlman      = "pyflowx.cli.emlmanager:main"
 envdev      = "pyflowx.cli.envdev:main"
 envpy       = "pyflowx.cli.envpy:main"