fix(bd): show 'no children' message when issue has no children

The --children flag would produce no output for issues without children.
Initialize the map entry in processIssue so the display loop can show
"No children found" message.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/show.go

@@ -912,6 +912,11 @@ func showIssueChildren(ctx context.Context, args []string, resolvedIDs []string,
 
 	// Process each issue to get its children
 	processIssue := func(issueID string, issueStore storage.Storage) error {
+		// Initialize entry so "no children" message can be shown
+		if _, exists := allChildren[issueID]; !exists {
+			allChildren[issueID] = []*types.IssueWithDependencyMetadata{}
+		}
+
 		sqliteStore, ok := issueStore.(*sqlite.SQLiteStorage)
 		if !ok {
 			// Fallback: try to get dependents without metadata