fix: drain SSH stdout/stderr while command runs to avoid large-output deadlock

[znick]

Before fix:

./cli -debug -hostname HOST -login LOGIN -devtype pc -command 'cat 2mb.txt' -password PASSWORD -json  > /dev/null
2026-01-27T00:03:06.936+0100	DEBUG	ssh/ssh.go:768	open connection	{"host": "HOST", "port": 22, "addresses": []}
2026-01-27T00:03:06.937+0100	DEBUG	ssh/ssh.go:1123	tcp dial	{"address": "HOST:22"}
2026-01-27T00:03:07.011+0100	DEBUG	ssh/ssh.go:1135	tcp ssh	{"address": "HOST:22"}
2026-01-27T00:03:07.364+0100	DEBUG	ssh/ssh.go:877	passwordCallback	{"passwordIndex": 0}
2026-01-27T00:03:07.459+0100	DEBUG	ssh/ssh.go:421	run cmd	{"cmd": "cat 2mb.txt"}

stuck forever, because nobody reads stdout

(2mb.txt - file with 2MB size)