PTY plugin protocol limits

Trap: validating only the plugin schema looks sufficient, but helper and framed-protocol limits can diverge and reject valid-looking sessions after authorization. Fix: keep all layers aligned at 128 arguments, 16,384 UTF-8 bytes per argument, 65,536 aggregate argument bytes, 3,600 runtime seconds, and 32,768-byte I/O payloads; split larger writes into bounded frames and validate canonical encoding.