chore: add AGENTS.md with Cursor Cloud development instructions

[ronros-elementor]

PR Checklist

• The commit message follows our guidelines: https://github.com/elementor/elementor/blob/master/.github/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

Summary

This PR can be summarized in the following changelog entry:

• Added AGENTS.md with Cursor Cloud specific development environment instructions for future cloud agents.

Description

Adds an AGENTS.md file containing Cursor Cloud specific instructions for setting up and working with the Elementor development environment. This includes:

• Prerequisites (Node.js 20.19, PHP >= 7.4, Composer)
• Key development commands (build, lint, test, watch)
• How to run the application locally via WordPress Playground
• Non-obvious gotchas discovered during setup (e.g., --ignore-scripts flag, turbo build ordering, PHP test Docker requirements)

Test instructions

This PR only adds documentation. No functional code changes.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended
• Docs have been added / updated (for bug fixes / features)