Problem

ruff ships in the [dev] extras but nothing runs it — installed-but-unenforced, so findings accumulate silently. Measured today (2026-06-12, default rule set):

Notable in the tail:

• Three scripts define a skip_langs set that is never used (scripts/check_skill_examples.py:178, scripts/check_readme_examples.py:75, scripts/check_examples_doc.py:75 — only check_spec_examples.py consumes its copy). Dead code that looks load-bearing.
• 5×F821 in tests/test_codegen_modules.py — string annotations referencing a type imported inside the function body. Harmless at runtime today, but precisely the kind of finding that rots into a real bug.

Plan

1. ruff check --fix the 136 auto-fixables; manually triage the rest, including deleting the dead skip_langs sets.
2. Add ruff check (default rules) to .pre-commit-config.yaml and the CI lint job.
3. Default rules only for now — rule-set expansion (pyupgrade, isort, etc.) is a separate later conversation.

Audit reference: §0.1 and Open Question 1 (enforce the tools you install). Size: S.