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/memory/memory.go

@@ -827,6 +827,32 @@ func (m *MemoryStorage) ClearDirtyIssuesByID(ctx context.Context, issueIDs []str
 	return nil
 }
 
+// ID Generation
+func (m *MemoryStorage) GetNextChildID(ctx context.Context, parentID string) (string, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Validate parent exists
+	if _, exists := m.issues[parentID]; !exists {
+		return "", fmt.Errorf("parent issue %s does not exist", parentID)
+	}
+
+	// Calculate depth (count dots)
+	depth := strings.Count(parentID, ".")
+	if depth >= 3 {
+		return "", fmt.Errorf("maximum hierarchy depth (3) exceeded for parent %s", parentID)
+	}
+
+	// Get or initialize counter for this parent
+	counter := m.counters[parentID]
+	counter++
+	m.counters[parentID] = counter
+
+	// Format as parentID.counter
+	childID := fmt.Sprintf("%s.%d", parentID, counter)
+	return childID, nil
+}
+
 // Config
 func (m *MemoryStorage) SetConfig(ctx context.Context, key, value string) error {
 	m.mu.Lock()