add ruff linting

[lindestad]

Summary

This is stacked on top of #30 and should be reviewed after the uv migration shape is agreed.

• adds Ruff as a uv lint dependency group
• configures Ruff formatting and a conservative lint rule set
• adds a Checks workflow that runs Ruff on pull requests and pushes to main
• applies Ruff formatting and the small lint fixes required by the new gate

Stack

Base PR: #30, uv-migration -> main

This PR: ruff-lint -> uv-migration

Once #30 lands, this PR can be retargeted to main or rebased onto main if GitHub does not update it cleanly.

Validation

• uv run --frozen --group lint ruff format --check .
• uv run --frozen --group lint ruff check .
• uv run --frozen --group dev pytest -> 3 passed