The witness manager's Stop() method was only updating runtime JSON state without killing the tmux session, causing 'gt rig shutdown' to leave witness sessions running. Added sessionName() method and tmux kill-session logic to match the refinery's existing implementation. Fixes: bd-gxaf 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>