refactor: drop unused tool schema defaults

docs/core-data-structures/tools.md

@@ -57,8 +57,6 @@ interface SchemaProp {
   description?: string
   /** Enum of allowed values (strings only). */
   enum?: string[]
-  /** Default value. */
-  default?: unknown
   /** Nested properties for type: 'object'. */
   properties?: SchemaSpec
   /** Items schema for type: 'array'. */