Implement hierarchical child ID generation (bd-171)

- 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

internal/storage/storage.go

@@ -65,6 +65,9 @@ type Storage interface {
 	GetJSONLFileHash(ctx context.Context) (string, error)
 	SetJSONLFileHash(ctx context.Context, fileHash string) error
 
+	// ID Generation
+	GetNextChildID(ctx context.Context, parentID string) (string, error)
+
 	// Config
 	SetConfig(ctx context.Context, key, value string) error
 	GetConfig(ctx context.Context, key string) (string, error)