* feat(chatmodes): add Expert C++ Software Engineer mode and integrate into README
Introduce a concise, standards-driven C++ expert chat mode focused on maintainability, clarity, and evolving industry best practices.
- Add new chat mode file (C++ expert)
- Register in README with install badges
- Simplify guidance (avoid prescriptive details)
- Emphasize behavior-focused testing and legacy code strategies (Michael Feathers)
* apply "node update-readme.js"
