Unlike --prune which removes tombstones by age, --purge-tombstones removes
tombstones that have no open issues depending on them, regardless of age.
Also cleans stale deps from closed issues to tombstones.
Usage:
bd compact --purge-tombstones --dry-run # Preview what would be purged
bd compact --purge-tombstones # Actually purge
Note: Use --no-daemon to prevent daemon from re-exporting after cleanup.
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