When CreateTombstone was called on a closed issue, the CHECK constraint
(status = closed) = (closed_at IS NOT NULL) was violated because
closed_at was not cleared. Now setting closed_at = NULL in the UPDATE.
Added regression test for creating tombstone from closed issue.
Fixes: bd-fi05
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>