Break out stderr and stdout handling

[svevang]

My local ssh client is giving me an error banner:

** WARNING: connection is not using a post-quantum key exchange algorithm.
** This session may be vulnerable to "store now, decrypt later" attacks.
** The server may need to be upgraded. See https://openssh.com/pq.html`

Because stdout and stderr are being mixed to test key handling, this is causing all kinds of globbing and weirdness when I execute the script.

This PR fixes the case where the messages show up in stderr but the ssh command does not exit.

• Captures SSH stdout and stderr separately
• Checks SSH exit status before processing remote environment output
• Preserves stderr handling for host-key failure recovery
• Only exports valid PRX_...=... environment lines
• Avoids unquoted expansion of mixed SSH output
• Prevents SSH warning banners from being interpreted as shell/export input