Add migration to convert existing issue fields to dependency edges:
- replies_to -> replies-to dependency with thread_id
- relates_to -> relates-to dependencies (JSON array)
- duplicate_of -> duplicates dependency
- superseded_by -> supersedes dependency
The migration is idempotent (INSERT OR IGNORE) so it does not duplicate
edges that were already created by Phase 2 dual-write.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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