docs: add AGENTS.md with Cursor Cloud setup notes

[cb-vishaal]

What

This repository is Chargebee's GitHub organization profile repo — its only content is profile/README.md (plus a top-level README.md symlink) that renders on the org's GitHub landing page. There is no application code, package manager, build system, tests, or services.

This PR adds an AGENTS.md capturing durable, non-obvious guidance for future Cursor Cloud agents so they don't waste time searching for a build/test system that doesn't exist, and documents how to preview the rendered Markdown locally.

Environment setup outcome

• Dependencies: none — the startup update script is a no-op.
• Hello-world demonstration: rendered profile/README.md to HTML and viewed it in Chrome to confirm the profile page displays correctly (all sections and links).

chargebee_profile_readme_preview.mp4

Rendered README top
Rendered README bottom

Notes

• No code was modified; only AGENTS.md was added.
• Please merge the AGENTS.md changes so future agents remember this repo is documentation-only.

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