Add a new --prune mode to `bd compact` that removes expired tombstones from
issues.jsonl without requiring AI compaction or deleting closed issues.
Features:
- `bd compact --prune` removes tombstones older than 30 days (default TTL)
- `bd compact --prune --older-than N` uses custom N-day TTL
- `bd compact --prune --dry-run` previews what would be pruned
- Supports --json output for programmatic use
This reduces sync overhead by eliminating accumulated tombstones that were
previously only pruned as a side effect of compaction or cleanup operations.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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