Add CLI version and distribution identity reporting

[stef-k]

Closes #91

Summary

• Add deterministic code-guard --version and code-guard --version --json fast paths backed by installed distribution metadata.
• Normalize unavailable, unreadable, and invalid metadata through the existing exit-3 error channels.
• Preserve checkout-runner parity without configuration, scope, Git, provider, skill, or guard work.
• Document the CLI contract and introduce a maintainable CHANGELOG.md Unreleased section.

Validation

• python -m unittest tests.test_cli_version — 16 passed.
• python -m unittest discover -s tests — 286 passed.
• Fresh built-wheel suite in an isolated environment — 286 passed.
• Installed-wheel human, JSON, and help probes from an unrelated directory — passed.
• Wheel contents and console entry point inspection — passed.
• git diff --check main...HEAD — passed.
• Code Guard exact changed-file review — only the unchanged pre-existing print_text complexity REVIEW.