Tests findRepliesTo() and findReplies() helper functions in cmd/bd/show.go
that traverse message threads via dependencies. Covers:
- 3-message linear thread chain traversal (up and down)
- Thread root finding via repeated findRepliesTo() calls
- Branching threads (one message with multiple replies)
- Empty/standalone messages
- Nonexistent issue IDs
- Verification that only replies-to deps are followed
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
