Add AGENTS.md with Cursor Cloud development environment setup

[reservelab]

What

Adds AGENTS.md with Cursor Cloud specific instructions for developing in this documentation-only codebase.

Why

This file provides future cloud agents (and developers) with essential context about:

• How to lint the Markdown and YAML files
• How to validate the skill's internal file references
• How to test the skill by installing it into a target project
• The key file structure and the fact that there is no build step or runtime

Changes

• AGENTS.md — New file with overview, linting commands, testing guidance, build/run notes, and key file structure reference.

Environment Setup

• Update script: npm install -g markdownlint-cli — installs the only development tool needed for this repo.
• Linting: markdownlint '**/*.md' --ignore node_modules for Markdown, python3 -m yamllint .reservelabs.example.yml for YAML.
• No build/server: This is a documentation-only Claude Code skill suite with no executable code.