Service compose and .env content was sent inline in the SSH command.
Linux limits one argument to 128 KiB, so a service with a generated
compose over about 96 KB failed to save or start with "Argument list
too long".
- Add instant_remote_write_file(), which streams content to a remote
tee over SSH stdin (sudo tee for non-root SSH users)
- Add SshMultiplexingHelper::generateSshStdinCommand() with the same
SSH options as generateSshCommand()
- Write the service compose and .env with the new helper