Pin ruff format settings explicitly in pyproject.toml

[dgenio]

Context

The docs claim ruff format uses "double quotes, trailing commas" (workflows.md § Code style), which is currently accurate — these are ruff's defaults. However, there is no explicit [tool.ruff.format] section in pyproject.toml to pin these settings.

If ruff changes its defaults in a future release, the docs would silently become inaccurate.

Proposed change

Add an explicit [tool.ruff.format] section to pyproject.toml:

[tool.ruff.format]
quote-style = "double"
skip-magic-trailing-comma = false

This makes the contract explicit and resilient to upstream default changes.

Origin

Raised during review of #94 (docs rewrite PR).