Read-only mounts (DMG images like /Volumes/Ghostty, update snapshots) with
~0% free produced false-positive low-storage alerts. StorageVolumeContext
now carries isReadOnly and totalBytes; the rule skips read-only volumes and
volumes under 1 GB where thresholds are meaningless. Regression tests cover
both exclusions plus the normal trigger path.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>