feat(config): Add AI assistant instruction files from mathemage/mathemage

[mathemage]

Copy over AI assistant configuration files from https://github.com/mathemage/mathemage:

• AGENTS.md - AI coding assistant instructions
• .github/copilot-instructions.md - GitHub Copilot instructions
• CLAUDE.md - Claude Code instructions
• .cursorrules - Cursor instructions
• .windsurfrules - Windsurf instructions

These files define coding standards, Git/GitHub workflow, and commit message conventions.