- sshmanager/deploy.go: StdinPipe() must be called BEFORE Start(),
not after. Reordered the calls to fix "ssh: StdinPipe after
process started" error.
- Return DeployResult instead of nil error on SSH dial failure
so the frontend always gets a parseable response.
- Add slog.Debug for key upload, slog.Warn for ssh-keyscan and
authorized_keys failures, slog.Info for final result summary.