Add PLAN.md and .gitignore

PLAN.md is the single source of truth for the CHIP for Magento module
implementation. It captures:

- Whether Magento is still active (yes, 2.4.x line is current)
- Repository strategy: one repo, one code base, branch by Magento version
- Full target code structure (controllers, gateway commands, models, etc.)
- Integration architecture: redirect-based flow, webhook as source of truth
- Payment method priority list (DuitNow QR, FPX, MPGS, etc.)
- Anti-patterns to avoid
- Phased delivery plan (MVP, expansion, inline methods)
- Reference plugins to study (Stripe, Adyen, Razorpay, Billplz, PayPal)
- Open questions for the team

Intended to be read by any future AI agent or developer before they write
code, and updated as decisions evolve.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: wzul

.gitignore

@@ -0,0 +1,21 @@
+# PHP / Composer
+/vendor/
+composer.lock
+*.phar
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+.DS_Store
+
+# Tests / build artifacts
+/build/
+/tmp/
+*.log
+/var/
+
+# CI
+.phpunit.result.cache
+.phpunit.cache