fix: expose the raw SDK error on AiError.cause - #16

Since 7.1.0, AiError carries a typed reason instead of message/cause, so AiError.cause is undefined. Consumers that walk .cause chains generically to find statusCode / responseBody (e.g. mockingbird's engine) stop finding them and have to special-case instanceof AiError.

This adds override get cause() returning this.reason.cause (the raw SDK error, after RetryError unwrapping). It is a prototype getter, not a constructor field: Effect's error constructor does not set an own cause, so nothing shadows it. Additive, shipped as a patch.

Tests: cause is the same object as reason.cause, is not an own property, and a generic .cause walker reaches statusCode and responseBody from an APICallError.