docs: add CLAUDE.md documenting project structure, test fixtures, and how to run tests

[jimisola]

Problem

The repository has no documentation explaining:

• How the test fixtures work
• How to run tests and what each test validates
• How to regenerate expected output fixtures when plugin output changes intentionally

The CONTRIBUTING.md only covers setup and ./gradlew build with no project-specific content.

Proposed solution

Add a CLAUDE.md (following the Claude Code convention) covering:

• Project structure and what the Gradle plugin/task does
• Test structure and what each test validates
• Fixture layout and what each fixture represents
• How to regenerate expected output fixtures when plugin behaviour changes intentionally
• How to build and run tests (./gradlew build, ./gradlew test)