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 in src/test/resources/ work (YAML annotation fixtures, ZIP assembly fixtures)
• How to run tests and what each test validates
• How to regenerate expected output fixtures (e.g. yml/combined_annotations.yml) when plugin output changes intentionally

The CONTRIBUTING.md only links to the org-wide contributing guide with no project-specific content.

Proposed solution

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

• Project structure and what the Mojo does
• Test structure: what testCombine() and testAssembleZipArtifact() test
• Fixture layout in src/test/resources/ and what each fixture represents
• How to regenerate combined_annotations.yml when the merge logic changes
• How to build and run tests (mvn clean verify)