Standardise on black for formatting, ruff for linting

[monkeypants]

Problem

The project uses black for formatting and ruff for linting, but this isn't
explicit anywhere. Both tools are in dev dependencies, and ruff format --check
reports 22 files diverging from ruff's formatting opinions — even though black
(the actual project formatter) is happy with all files.

This ambiguity could lead a new contributor to run ruff format instead of
black, introducing unnecessary churn.

Proposal

• Document explicitly that black is the formatter, ruff is the linter
• Add a [tool.ruff.format] section or comment in pyproject.toml clarifying
  that ruff format is not used
• Ensure the CI doctrine job uses uv run consistently (it currently calls
  pytest directly, which can pick up the wrong Python)

Why black over ruff format

• Michael is actively using black — match the workflow of the person writing code
• black is PSF-ecosystem, maintained by CPython core developers
• ruff format has minor opinion divergences from black that cause false alarms
• Speed difference is irrelevant at this codebase size
• Keeps ruff focused on what it's best at: fast linting

Closes #126