chore(reporting): use importlib to load themes and assets

Replace Path(file)-based resource loading with importlib.resources throughout the reporting module. generate_output.py now uses as_file() context managers for the Lua filter and Pandoc HTML template so pandoc always receives a real filesystem path.

_configure_output_directory uses files() + a recursive _copy_resource helper instead of shutil + file. utilities.py uses files() to locate the output_dir template path.

Closes #341 (closed)

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com