feat: debug --sshd flag, auto SSH + nc listener + IP callback

When using `labctl provision debug <target> --sshd`, the rescue
kickstart generates host keys, starts sshd (pw: debug) and nc
listener (port 2323), and reports the IP back to bastion via
/api/progress callback. Fully self-contained, no mounted FS needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bastion/completions/labctl.fish

@@ -122,6 +122,7 @@ complete -c labctl -n "__labctl_in_cmd init bastion standalone stop" -l dir -d '
 complete -c labctl -n "__labctl_using_cmd provision" -a list -d 'List all known machines'
 complete -c labctl -n "__labctl_using_cmd provision" -a install -d 'Queue a discovered machine for OS installation'
 complete -c labctl -n "__labctl_using_cmd provision" -a reprovision -d 'Queue install + SSH reboot into PXE (target: hostname, MAC, or IP)'
+complete -c labctl -n "__labctl_using_cmd provision" -a debug -d 'PXE boot into Fedora rescue mode for debugging (target: hostname, MAC, or IP)'
 complete -c labctl -n "__labctl_using_cmd provision" -a forget -d 'Remove a machine from bastion state'
 complete -c labctl -n "__labctl_using_cmd provision" -a logs -d 'Show provisioning logs for a machine (hostname, MAC, or IP)'
 complete -c labctl -n "__labctl_using_cmd provision" -a makeiso -d 'Generate a UEFI-bootable iPXE ISO for network provisioning'