Add llms.txt to repo root for LLM context

[GarthDB]

Part of #266 — Leonardo AI tooling initiative.

Summary

Add a llms.txt file at the repository root so that LLMs and AI coding assistants can quickly orient themselves to Leonardo's purpose, package structure, and API without reading source files.

Why

llms.txt is a lightweight convention (similar to robots.txt) that gives LLMs a structured starting point. Spectrum Design Data uses this same pattern: https://opensource.adobe.com/spectrum-design-data/ai/

What to include

• Project description and primary use case (contrast-based adaptive color)
• Monorepo structure (packages/contrast-colors/, docs/ui/)
• Public API summary: Theme, Color, BackgroundColor, createScale, contrast, convertColorValue, luminance
• Supported colorspaces for interpolation (LCH, LAB, CAM02, HSL, HSLuv, HSV, RGB, OKLAB, OKLCH)
• Supported output formats (HEX, RGB, HSL, HSV, HSLuv, LAB, LCH, OKLAB, OKLCH, CAM02, CAM02p)
• Contrast methods (wcag2, wcag3/APCA)
• Links to the npm package and the web app at leonardocolor.io

Acceptance criteria

• llms.txt exists at repo root
• Covers all major API exports with brief descriptions
• Accurate colorspace and output format lists
• Links to npm, GitHub, and leonardocolor.io