Root cause: The merge driver matched issues by IssueKey (ID+CreatedAt+CreatedBy).
When timestamp precision differed (e.g., with/without nanoseconds), the same
issue was treated as two different issues, causing both tombstone and closed
versions to appear in the merge result.
Fix: Added ID-based fallback matching in merge3WayWithTTL. When key-based
matching fails but the same ID exists in the other side, use that for merging.
Also track processed IDs to prevent duplicates.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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