Clear ruff lint backlog (167 errors), make CI lint blocking

[kbens]

The new CI workflow runs ruff check . with continue-on-error: true because the codebase has a 167-error backlog at the time v0.3.0 was cut:

83  E501  line-too-long
44  F401  unused-import           (auto-fixable)
18  F541  f-string-missing-placeholders (auto-fixable)
15  I001  unsorted-imports        (auto-fixable)
 5  F841  unused-variable
 1  E731  lambda-assignment
 1  F811  redefined-while-unused  (auto-fixable)

78 are auto-fixable with ruff check --fix. Then handle the line-too-long manually, then flip .github/workflows/test.yml to drop continue-on-error: true.