feat(cmd): add 'bd children <id>' command (bd-scbxh)

Convenience alias for 'bd list --parent <id>'. Lists all child beads
of a specified parent, supporting all standard list flags (--json,
--pretty, etc.).

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

cmd/bd/children.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"github.com/spf13/cobra"
+)
+
+// childrenCmd lists child beads of a parent.
+// This is a convenience alias for 'bd list --parent <id>'.
+var childrenCmd = &cobra.Command{
+	Use:     "children <parent-id>",
+	GroupID: "issues",
+	Short:   "List child beads of a parent",
+	Long: `List all beads that are children of the specified parent bead.
+
+This is a convenience alias for 'bd list --parent <id>'.
+
+Examples:
+  bd children hq-abc123        # List children of hq-abc123
+  bd children hq-abc123 --json # List children in JSON format
+  bd children hq-abc123 --pretty # Show children in tree format`,
+	Args: cobra.ExactArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		parentID := args[0]
+
+		// Set the parent flag on listCmd, run it, then reset
+		listCmd.Flags().Set("parent", parentID)
+		defer listCmd.Flags().Set("parent", "")
+		listCmd.Run(listCmd, []string{})
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(childrenCmd)
+}