chore(python): audit tests

Split the three oversized test files into support modules and topic
files, evaluate the rmdir dir_fd capability once at import, and drop
unused imports.

Claude-Session: https://claude.ai/code/session_01Agoc9bRVE9KL7mmWtey9Vq

tests/test_kirocli_plugin.py

@@ -5,7 +5,6 @@ import json
 import os
 from pathlib import Path
 import subprocess
-import sys
 from typing import Any
 
 import pytest