Fix sync bug where newly created issues were incorrectly tombstoned during bd sync.
The root cause was git-history-backfill finding issues in local commits on the sync branch, then tombstoning them when they weren't in the merged JSONL. The fix protects issues from the left snapshot (local export) from git-history-backfill.
🤖 Generated with [Claude Code](https://claude.com/claude-code)