fix: enforce repo hygiene for backup artifacts

[TATP-233]

Summary

• remove tracked .bak backup assets from the allegro hand source tree
• ignore common backup and temporary file suffixes in .gitignore
• add a pytest-backed repository hygiene check and document the workflow

Validation

• uv run pytest tests/scripts/test_repo_hygiene.py -q
• uv run pytest tests/scripts/test_check_docs.py -q
• make check
• make test

Impact Scope

• repository hygiene
• contributor workflow docs
• CI-covered pytest checks

Fixes #170