Rewrites python_pkg.diet_guard imports to diet_guard, vendors the
shared as_float coercion helper, drops the monorepo PYTHONPATH from
install.sh and the systemd unit (package is now pip-installed), and
scaffolds standalone lint/test config matching testsAndMisc's real
enforced bar (pylint --fail-under=10 with tests excluded and the
use-implicit-booleaness/consider-using-with disables, mypy's actual
disabled-error-code set, ruff ALL, bandit, 100% branch coverage).
