Add PLAN.md — initial implementation plan

[wzul]

What does this change?

This PR adds PLAN.md, the single source of truth for the CHIP for Magento 2 payment module implementation. It also adds a .gitignore for a typical Magento module repo.

PLAN.md captures:

• Magento lifecycle context (still active in 2.4.x; we target the 2.4.x code line)
• Repository strategy: one repo, one code base, branches by Magento version (NOT one-repo-per-version)
• Full target file/directory structure (controllers, gateway commands, models, etc.)
• Integration architecture: redirect-based flow, webhook as source of truth, idempotency
• Payment method priority list (DuitNow QR + FPX first; MPGS, wallets, GPay/ApplePay later)
• Anti-patterns to avoid
• Phased delivery plan (MVP → expansion → inline methods)
• Reference plugins to study (Stripe, Adyen, Razorpay, PayPal)
• Open questions for the team

Why a doc-only PR first?

Before writing any code, the team (and any future AI agent) needs to agree on the architecture and resolve the open questions. This doc is the starting point for that conversation. Each open question in section 10 should get answered (or marked resolved with a date/note) before we move to Phase 1 of section 11.

Reviewers — please check

1. Do the open questions in section 10 ring true? Anything missing?
2. Is the payment method priority list in section 6 right for our go-to-market?
3. Is the phased delivery plan in section 11 realistic given team capacity?
4. Should the Composer package name be chipin/chip-for-magento (current draft) or something else?

After merge, the next PR will be Phase 0: composer.json, registration.php, module.xml, and CI.

How to test

This is a documentation-only change. No code, no tests, no deployment.

PR Checklist

• Unit test provided? — N/A (docs only)
• All unit tests passed? — N/A (docs only)
• Deployed and tested in staging? — N/A (docs only)
• Project Management Solution task link provided? — needs ticket
• Is this dependent on the Back-End / Front-End? — no