docs,core: refresh README/test docs and tighten rig verdict parsing

- rebuild README quick start, feature, and status sections around the llm-guard-cli binary and current provider workflow
- sync the testing guide with 53 unit / 69 total tests, add snapshot & TLS builder guidance, and update macOS TLS troubleshooting
- factor rig verdict extraction into verdict_from_choice, reusing the existing logging/sanitising flow and covering fallback cases with unit tests
- reference the Anthropic rig screenshot in the usage guide for completeness

Author: HendrikReh