docs: Add AI coding assistant configuration files

[youknownix]

Summary

• Add CLAUDE.md for Claude Code integration
• Add AGENTS.md for general AI coding assistants (Cursor, GitHub Copilot, etc.)

What these files provide

Both files give AI assistants project-specific context:

Section	Description
Project overview	Core principles (web-native, server-rendered, zero dependencies)
Development commands	shopify theme check, npm run build:schemas, shopify theme dev
Architecture	Directory structure, component hierarchy (sections → blocks → snippets)
Critical rules	Conditional schema editing workflow, single content_for 'blocks' rule, snippet documentation
Code standards	Liquid, JavaScript, CSS, and accessibility guidelines
Key files	Component base class, global styles, settings schema locations

Why two files?

• CLAUDE.md — Recognized by Claude Code CLI, Anthropic's official coding assistant
• AGENTS.md — Generic convention recognized by multiple AI coding tools (Cursor, Windsurf, etc.)

Both files have identical content to ensure consistent behavior across different AI tools.

Benefits for contributors

• Reduces onboarding time for AI-assisted development
• Prevents common mistakes (e.g., editing schemas directly when a build system exists)
• Ensures AI-generated code follows Horizon's conventions
• Improves PR quality and reduces review cycles

No impact on theme publishing

These root-level markdown files are excluded from theme packages:

• shopify theme package only includes standard theme directories: assets/, blocks/, config/, layout/, locales/, sections/, snippets/, templates/
• Files outside these directories are not uploaded to Shopify
• No effect on merchant stores, theme functionality, or performance
• No changes to theme review eligibility

References

• Claude Code documentation
• Shopify theme directory structure
• Theme package CLI command

Test plan

• shopify theme check passes with no new errors
• shopify theme package generates ZIP without these .md files
• Existing theme functionality unaffected