Run or Debug on a Python file started it as a script
(`python src/tasks/cli.py`), so a module of a package that imports
relatively (`from .models import TaskList`) always failed with
"attempted relative import with no known parent package".
A file with relative imports now runs as `python -m package.module`
from the directory above its top package (the enclosing directories
with `__init__.py`, read once and re-read when one is added or
removed), with a venv-relative interpreter resolved against the project
root. Files without relative imports still run as scripts.
