add rdk x5 yolo skills

[Machao615]

Summary

• Add rdk-yolo-toolkit for end-to-end YOLO training, ONNX export, RDK X5 BPU quantization, board inference, and TROS/ROS2 deployment.
• Add rdk-x5-toolchain-quantization for model-agnostic OpenExplorer PTQ workflows.
• Update the README skill list.

Validation

• quick_validate.py skills/rdk-yolo-toolkit
• quick_validate.py skills/rdk-x5-toolchain-quantization

Note: node scripts/validate-skills.mjs currently reports pre-existing baseline frontmatter issues in existing skills.