Make shell command graders platform-aware

[paulcam206]

Summary

• select PowerShell on Windows and bash on POSIX without relying on shell=True
• keep git fixture setup platform-independent and surface missing shell infrastructure
• normalize relative, absolute, and backslash-delimited command output paths
• execute both embedded graders in behavior tests, including valid and invalid candidates

Testing

• python -m pytest tests/test_task_shell_graders.py -q (10 passed, 6 subtests)
• python -m pytest -q (42 passed, 6 subtests)
• ruff check tests/test_task_shell_graders.py
• python scripts/lint_argparse_help.py
• python scripts/lint_manifest.py

Repo-wide ruff check . still reports pre-existing violations outside the changed files.