docs: add CLAUDE.md documenting project structure, test types, and fixture usage

[jimisola]

Problem

The repository has no documentation explaining:

• How the fixture project in tests/fixtures/test_project/ works and why it exists
• How to run unit tests vs e2e tests separately
• The pre-requisite that a wheel must be built (hatch build --target wheel) before e2e tests can run
• How to regenerate expected output fixtures when plugin output changes intentionally

The existing CONTRIBUTING.md only covers setup and hatch build / hatch run test.

Proposed solution

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

• Project structure
• How to run unit tests vs e2e tests (pytest -m e2e vs default)
• What the fixture project is and how it is structured
• Pre-requisites for e2e tests (build wheel first)
• How to regenerate expected output fixtures