feat(branding): add official KERNL logos and branding guide

- Added 3 logo variations:
  - kernl-logo-light.png (transparent background, light theme)
  - kernl-logo-dark.png (dark theme optimized)
  - kernl-icon-light.png (icon version for small sizes)

- Updated BRANDING_GUIDE.md with comprehensive guidelines:
  - Logo usage rules (spacing, sizes, do's/don'ts)
  - Color palette (cyan-purple gradient, red accent)
  - Brand voice and messaging
  - Platform-specific branding
  - Typography recommendations
  - Launch messaging templates

- Removed LOGO_TODO.md (logos now complete)

Logo design:
- Hexagonal shape (structure/foundation metaphor)
- Cyan-to-purple gradient border (modern AI aesthetic)
- Dark/white "K" letterform (readable, professional)
- Red accent dot (the "kernel" - core intelligence)

Tagline: "The Core Intelligence Layer for AI Systems"