Add viget-brand-guidelines skill

[joshuapease]

Summary

• Adds the viget-brand-guidelines skill defining Viget's complete brand identity: colors, typography, buttons, layout, logo, voice/tone, presentation styling, photography, and data visualization guidance
• Covers both web and presentation contexts with clear distinctions where they differ (e.g., heading weights)
• Includes inline SVG of the Viget logo for contexts that support it

Closes #1

Test plan

• Install the skill and verify it triggers on brand-related prompts (e.g., "use Viget brand colors", "style this like Viget")
  • You can download the file by clicking here and saving the markdown file.
  • View https://github.com/vigetlabs/viget-agent-skills/blob/main/docs/using-skills-in-claude-web-and-desktop.md for installation instructions
• Confirm color values, typography specs, and voice guidelines match viget.com
• Test presentation-specific rules are correctly applied when generating slides
• Verify the skill stays under the 500-line SKILL.md limit