Better handling of tasks without actions

[bollwyvl]

Elevator Pitch

Allow for tasks with sub-tasks to define meta, task_dep, without actions.

Motivation

The primary use case is for allowing skipping whole chunks of tasks with meta, such as not running formatters in CI:

# pyproject.toml
[tool.doitoml.tasks.format]
meta = {doitoml={skip="${CI}"}}

[tool.doitoml.tasks.format.black]
# ...

[tool.doitoml.tasks.format.ruff]
# ...

Design Ideas

TBD