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/create.go

@@ -77,7 +77,7 @@ var createCmd = &cobra.Command{
 		}
 
 		// Get field values, preferring explicit flags over template defaults
-		description, _ := cmd.Flags().GetString("description")
+		description, _ := getDescriptionFlag(cmd)
 		if description == "" && tmpl != nil {
 			description = tmpl.Description
 		}