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

tests/cli/test_which.py

@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import shutil
-from pathlib import Path
 from unittest.mock import patch
 
 import pytest
@@ -12,45 +11,6 @@ import pyflowx as px
 from pyflowx.cli import which
 
 
-# ---------------------------------------------------------------------- #
-# which_command
-# ---------------------------------------------------------------------- #
-class TestWhichCommand:
-    """Test which_command function."""
-
-    def test_returns_path_when_command_found(self, capsys: pytest.CaptureFixture[str]) -> None:
-        """Should return Path when command is found."""
-        with patch.object(shutil, "which", return_value="/usr/bin/python"):
-            result = which.which_command("python")
-            assert result == Path("/usr/bin/python")
-            captured = capsys.readouterr()
-            assert "匹配路径" in captured.out
-            assert "/usr/bin/python" in captured.out
-
-    def test_returns_none_when_command_not_found(self, capsys: pytest.CaptureFixture[str]) -> None:
-        """Should return None when command is not found."""
-        with patch.object(shutil, "which", return_value=None):
-            result = which.which_command("nonexistent_cmd")
-            assert result is None
-            captured = capsys.readouterr()
-            assert "未找到" in captured.out
-            assert "nonexistent_cmd" in captured.out
-
-    def test_prints_match_path_on_success(self, capsys: pytest.CaptureFixture[str]) -> None:
-        """Should print '匹配路径: - <path>' on success."""
-        with patch.object(shutil, "which", return_value="C:\\Python\\python.exe"):
-            _ = which.which_command("python")
-            captured = capsys.readouterr()
-            assert "匹配路径: - C:\\Python\\python.exe" in captured.out
-
-    def test_prints_not_found_on_failure(self, capsys: pytest.CaptureFixture[str]) -> None:
-        """Should print '<command>: 未找到' on failure."""
-        with patch.object(shutil, "which", return_value=None):
-            _ = which.which_command("missing")
-            captured = capsys.readouterr()
-            assert "missing: 未找到" in captured.out
-
-
 # ---------------------------------------------------------------------- #
 # main function
 # ---------------------------------------------------------------------- #