The close_reason and closed_by_session fields were being silently dropped
during 3-way merge operations because the simplified Issue struct in
internal/merge/merge.go was missing these fields.
Changes:
- Add CloseReason and ClosedBySession fields to merge.Issue struct
- Implement merge logic that preserves these fields when status is closed
- Use timestamp-based conflict resolution (later closed_at wins)
- Clear close metadata when status becomes non-closed
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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