feat(icons): Add prompting skill icon, update voice system icon

- New icon for kai-prompting-skill (nested brackets motif)
- Refreshed icon for kai-voice-system (speaker with sound waves)
- Both 256x256 RGBA PNGs with transparent backgrounds

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: danielmiessler