Add AGENTS.md

[slogsdon]

Description

Adds an AGENTS.md context file to help AI coding agents (GitHub Copilot, Cursor, etc.) understand the project structure, setup steps, environment variables, test cards, and API conventions for this sample project.

Changes Made

• Added AGENTS.md with project overview, architecture, setup instructions, environment variable reference, test card data, and contribution guidelines

Testing

• Manual testing completed

[theCreedo]

Overall this reads well — the critical patterns section (SHA512 token gen, AUT_ prefix stripping, 3DS field replay) are exactly what a fresh agent needs to not break the auth flow, and the .env.example vs .env.sample callout is the right thing to lead with.

Two things to fix before merging:

Line 87 — developer.globalpay.com is the legacy domain. Canonical URL is developer.globalpayments.com.

Line 112 — hardcoded machine path (/Users/shane.logsdon/Code/temp/gpapi-3ds2). Swap with $PWD or cd /path/to/gpapi-3ds2 so it works on any clone.

One thing to note but not block on: php/run.sh doesn't pass the router.php argument, so PHP routes will break if someone follows the AGENTS.md instruction and runs ./run.sh instead of the manual command. Worth a separate fix on main.