Make pyrefly error on unused ignore comments (#1898)

Pyrefly's unused-ignore rule is silent by default, so stale
`# pyrefly: ignore` comments go unnoticed after upgrades.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

Author: adamtheturtle

pyproject.toml

@@ -387,6 +387,7 @@ plugins = [
 
 [tool.pyrefly]
 errors.non-exhaustive-match = "error"
+errors.unused-ignore = "error"
 
 [tool.pyright]
 typeCheckingMode = "strict"