enhancement: support Windows testing and terminal behavior

[codeforester]

Summary

• add a cross-platform shell identifier with a COMSPEC fallback on Windows;
• make terminal detection tolerate streams that raise ValueError from isatty();
• keep color and redirected-output behavior deterministic;
• document the Windows-aware test environment variables;
• add regression coverage for shell, terminal, and output behavior.

Fixes #38

Validation

• uv run --no-project --with pytest --with click --with PyYAML python -m pytest
• ./tests/validate.sh

111 tests passed.