howells-oxfmt --stdin-filepath returned an empty string and exit 0: every
wrapper runs its tool through spawnPackageBinCapture, which captures stdio
and so hands the child no stdin. A stdin run now inherits the parent's
stdio and passes the tool's stdout through unclassified. howells-oxlint,
howells-check and howells-fix cannot serve stdin and now refuse it with a
one-line message and exit 2.