Add CLAUDE.md with build commands and architecture guidance

[iliyan-velichkov]

Summary

Adds a CLAUDE.md file to give Claude Code (and human contributors) a fast onboarding to this repository.

It documents:

• The big picture — Rhea is a thin codbex "Edition" that packages Eclipse Dirigible into a runnable Spring Boot app; most feature work happens at the Maven dependency/packaging layer, not in Java code. Rhea's focus is Entities, Forms and Reports modeling for Model Driven Architecture (see https://www.codbex.com/products/).
• Build, run, test, and format commands, including the Maven profiles (quick-build, unit-tests, integration-tests, tests, format) and how to run a single test.
• The customization model — how stock Dirigible UI components are overridden via exclusions + local components/ui/ replacement modules.
• Module layout, the parent-POM inheritance chain, Spring profile activation, and Docker notes.

Test plan

• Documentation-only change; no build or runtime impact.

🤖 Generated with Claude Code