Disconnect the one-shot accounts command from usageSource after parsing
its output. usageSource polls on an interval, so leaving the source
connected made it re-spawn the codexbar CLI every refresh forever after
each "Accounts" click, with its output silently discarded.

Route update-widget.sh I/O failures through fail() so the most common
runtime errors (offline, GitHub 404/rate-limit, interrupted download,
install/permission errors) emit the structured JSON error the widget
expects, instead of aborting under set -e with empty stdout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>