tlog PTY merges stderr into stdout for non-interactive SSH commands

## Problem

For session recording, remote commands are executed under a PTY, which is a single stream, so the command's stdout and stderr are merged. On hosts without enablement of session recording, SSH keeps stderr on a separate channel.

Reproduce:

ssh 'echo OUT; echo ERR >&2' # ERR still prints
ssh 'echo OUT; echo ERR >&2' # ERR suppressed

## Impact
Any tooling relying on separate streams is affected on managed hosts. One example is the error when enabling verbose on `sshuttle` command (#40)

Start by tracing how recorded remote commands are launched under a PTY and how their output reaches the SSH client. Reproduce the two SSH examples on recorded and unrecorded hosts, then verify that non-interactive commands preserve separate stdout and stderr without breaking session recording.

Written by the indexing model from the issue text.