chore: make clean should remove test artifacts

[adamdaw]

`make clean` and `make clean-all` remove generated PDFs/HTML/DOCX but do not clean test and coverage artifacts.

Files not cleaned

• `coverage/` (bashcov HTML + Cobertura XML)
• `.pytest_cache/`
• `pycache/`
• `luacov.stats.out` / `luacov.report.out` (if luacov is ever added)

Acceptance Criteria

• `make clean` removes coverage/ and .pytest_cache/
• Or add a dedicated `make clean-test` target
• Document in make help