Add AGENTS.md with Cloud Agent development instructions

[donvito]

Adds AGENTS.md with Cursor Cloud specific instructions for future agents working on this Electron-based Markdown Editor.

What's included

• How to run the app on Cloud Agent VMs (display server setup)
• Key caveats: DBus errors are non-fatal, no test suite, no linter, AI features require API config
• Build command references

Environment verification

The development environment was fully set up and verified:

• npm install installs all dependencies successfully
• npm start launches the Electron app
• npm run build:linux produces AppImage and .deb packages
• The editor was tested by opening sample.md and editing content with live preview

markdown_editor_hello_world_demo.mp4
Editor with Hello World content

_{To show artifacts inline, enable in settings.}