Add unreal-expert and qt-expert subagents

[PieterVI1]

Adds two new domain experts following the established agent format (Focus Areas / Approach / Quality Checklist / Output).

New agents

• unreal-expert - Unreal Engine C++: gameplay framework, UObject/GC lifetime, Slate/UMG UI, delegates, threading, and editor tooling.
• qt-expert - Qt and C++: Widgets, Qt Quick/QML, signals and slots, QObject ownership, model/view, and cross-platform builds.

Notes

• Both use model: claude-sonnet-4-20250514 and lowercase hyphenated names, consistent with existing agents.
• README updated: qt-expert under Mobile & Desktop Development, and a new Game Development section for unreal-expert.
• No overlap with the existing cpp-expert; these cover framework/engine-specific domains it does not.