Commit ruff format and ruff check --fix results separately in migration guide

@dwilding observed in #588 that it's handy to commit the `ruff format` results before diving into the deeper issues that `ruff check --fix` handles. Rather than advise running the underlying commands in the migration guide, let's add a `--no-fix` flag to `just format` which skips running `ruff check --fix`, and advise using that.

Start with the `just format` entry point and the migration guide mentioned in the issue. Check how `ruff format` and `ruff check --fix` are currently invoked, then verify that `--no-fix` skips only the latter and that the guide recommends it for separate commits.

Written by the indexing model from the issue text.