docs: add SKILL.md for Claude Code agent integration

Add a Claude Code skill file that provides comprehensive agent
instructions for using OpenEvolve. This makes the tool ready-to-use
for AI coding agents by documenting the full workflow: scaffolding
experiments, writing evaluators, configuring evolution, running jobs,
inspecting checkpoints, and retrieving evolved programs.

The skill file uses YAML frontmatter (name, description) so that
Claude Code can automatically discover and activate it when users
ask about evolutionary code optimization.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: DavyMorgan