configure.py did not parse its arguments, so `--help` printed no usage and
ran the bootstrap as a side effect (creating config.yaml, .env and
frontend/.env in a fresh checkout), and any unrecognized flag was silently
ignored the same way. Parse arguments with argparse before touching the
filesystem: --help now prints usage and exits 0, unknown flags exit 2, and
the no-argument invocation used by `make configure` is unchanged.
Extend the regression tests from #7102 with two offline subprocess cases
covering both new behaviors.
Fixes#7382