tests/conftest.py had grown to 2,343 lines, past the 2,000-line gate. Move
three self-contained topics out, unchanged:
- env_filter.py: the credential / behavioral env-var name tables the
hermetic fixture blanks;
- live_system_guard.py: the autouse live-system guard fixture, its marks
and the protected checkout roots;
- platform_gating.py: the platforms() marker evaluation used by the
collection hook.
conftest imports them instead of listing them in pytest_plugins: it is not
the rootdir conftest, and pytest fails a run that loads a non-root conftest
carrying pytest_plugins after startup. Imported fixtures register on the
conftest module under their old names, so autouse order is unchanged.
Tests that reached into the moved symbols now import the new modules.
