v0.8.21 — --stdin UTF-8 fix on Windows console

Fixed

• mcptoon call … --stdin no longer corrupts non-ASCII arguments on a Windows
  console. sys.stdin is bound to the active code page (cp936 on a Chinese UI,
  cp1252 elsewhere), so a UTF-8 JSON payload piped into --stdin was re-decoded
  through the wrong codec and sent to the server as mojibake — with no error, because
  the corrupted text is still valid JSON and json.loads accepts it. Both --stdin
  readers in _cmd_call now read sys.stdin.buffer and decode UTF-8 first, falling
  back to the stream's own encoding so a legacy console-piped payload still arrives
  unchanged. Reported and fixed by @GIGIGIGIbaby (#28).