- Add GetNextChildID to storage interface for generating child IDs
- Implement in SQLiteStorage with atomic counter using child_counters table
- Implement in MemoryStorage with in-memory counter
- Add --parent flag to bd create command
- Support hierarchical IDs (bd-a3f8e9.1, bd-a3f8e9.1.5) in CreateIssue
- Validate parent exists when creating hierarchical issues
- Enforce max depth of 3 levels
- Update ID validation to accept hierarchical IDs with dots
- Add comprehensive tests for child ID generation
- Manual testing confirms: sequential children, nested hierarchies, depth enforcement
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