Add pre-commit hook and composite action

[biggdawg320]

Closes #3.

Adds a reusable pre-commit hook and composite GitHub Action for the existing checker. Both preserve the distinction between a verified finding (exit 1) and NOT CHECKED (exit 2): findings fail the integration, while ordinary unmatched or unreadable files do not. Runtime failures still propagate from the Action.

Also adds the exit-code contract to the built-in self-test and documents both integrations in the README.

Validation run locally:

• PYTHONUTF8=1 python -m py_compile gwp_basis_check.py gwp_basis_check_hook.py
• PYTHONUTF8=1 python gwp_basis_check.py --self-test
• PYTHONUTF8=1 python render_tables.py --check
• PYTHONUTF8=1 python gwp_basis_check.py README.md
• wrapper/action-path checks for clean, NOT CHECKED, finding, and unmatched-path outcomes; YAML parse; git diff --check

The composite shell path was exercised locally with Git Bash plus the host Python shim. It has not run on hosted GitHub Actions yet.

AI-assisted implementation under Jim Hall's direction; scope, validation, and submission were reviewed by the project workflow.