@contextmanager cleanup code placed after yield doesn't execute on exception — the exception re-raises at the yield point, skipping everything below it. This left usethis_config in a corrupted state (e.g. quiet=True permanently) when exceptions propagated through with usethis_config.set(...) blocks.

Changes

• src/usethis/_config.py — Wrap yield in try-finally in UsethisConfig.set() so _restore() always runs
• src/usethis/_test.py — Same fix for change_cwd() so os.chdir(old_dir) always runs
• tests/usethis/test_config.py — Add tests verifying state restoration after exception
• .agents/skills/usethis-python-code/SKILL.md — Add this as a documented common mistake in the context manager guidance (v1.9)

# Before: cleanup skipped on exception
self.project_dir = project_dir
yield
self._restore(old)  # never reached if body raises

# After: cleanup guaranteed
self.project_dir = project_dir
try:
    yield
finally:
    self._restore(old)