add --help, auto-detect org, --window-days, read-only banner (#2048)
vrg-epic-audit now has an argparse CLI: --help documents that it audits the current repo's GitHub org (auto-detected from origin), scans merged PRs over a --window-days window (default 30), and is read-only. Promotes github._detect_org to public detect_org; the org is resolved from the repo instead of a hardcoded 'vergil-project', and an undetectable org now errors clearly instead of silently defaulting. render() prepends a banner naming the org/window and stating the run changed nothing — resolving the confusion that the output looked like a menu of actions. Removes vrg-epic-audit from the help-coverage KNOWN_GAPS ratchet. Ref vergil-project/vergil-tooling#2040.