Harden pre-commit safety hooks and document local checks

[jink-ucla]

Summary

Strengthens repository safety checks by expanding pre-commit hooks and documenting local usage for contributors.

Changes

• Update .pre-commit-config.yaml to include pre-commit-hooks:
  • check-yaml
  • detect-private-key
  • check-added-large-files (--maxkb=1000)
  • end-of-file-fixer
  • trailing-whitespace
• Keep existing formatting hooks (isort, black).
• Add a Pre-commit Checks section to CONTRIBUTING.md with install/run commands:
  • pip install pre-commit
  • pre-commit install
  • pre-commit run --all-files

Why

This reduces accidental secret leakage and catches common repo hygiene issues before PR review, while keeping contributor setup simple.

Validation

python -c "import yaml; yaml.safe_load(open('.pre-commit-config.yaml','r',encoding='utf-8')); print('pre-commit-yaml-ok')"