Add agent output size limit to prevent OOM

[chanakyav]

In agent.py, _stream_and_capture() writes all stdout into a StringIO with no size cap:

captured.write(decoded)  # Could accumulate GBs in StringIO

A runaway agent (e.g. accidentally printing a large binary file) could fill RAM.

Proposed fix:

1. Add a max_output_bytes config option (default: 50MB)
2. In _stream_and_capture(), check captured.tell() before writing
3. If limit exceeded, log a warning and stop capturing (but keep streaming to terminal)
4. Set the agent result to indicate truncation