feat: Add --body flag as alias for --description

Adds --body as a hidden alias for --description in create/update commands,
following GitHub CLI convention for better agent ergonomics.

The --body flag:
- Works alongside existing --description and -d flags
- Hidden from help output to avoid clutter
- Validates conflicts when both flags used with different values
- Available in both 'bd create' and 'bd update' commands

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/show.go

@@ -368,8 +368,8 @@ var updateCmd = &cobra.Command{
 			assignee, _ := cmd.Flags().GetString("assignee")
 			updates["assignee"] = assignee
 		}
-		if cmd.Flags().Changed("description") {
-			description, _ := cmd.Flags().GetString("description")
+		description, descChanged := getDescriptionFlag(cmd)
+		if descChanged {
 			updates["description"] = description
 		}
 		if cmd.Flags().Changed("design") {