DX-902: Add a non-ASCII guard with documentation and tests

[azeinela]

Summary

• add a detect-non-ascii-characters hook that strips or reports bytes outside printable ASCII + whitespace, with options for custom include ranges, extra allowed chars, file globs, and check-only mode
• document the new hook and register its console script entry
• add a fixture and comprehensive tests covering multibyte characters, filtering, globbing, and combined flag behaviors

Testing

• python -m pytest tests/non_ascii_guard_test.py

[rzuckerm]

We need to discuss this when Trevor gets back