fix(codex): don't treat a still-starting Codex as ready

Codex 0.157 draws its composer (dim placeholder, ready footer) before startup
finishes; input typed then is held as a startup draft and an onboarding
screen can still take over. hcom read the placeholder as an empty prompt and
declared launch ready immediately.

While the header reads `model: loading` or an onboarding footer
(`enter continue · esc quit`) is visible, Codex input is unknown and the
screen is not ready. A quiet loading screen is also no longer taken as a
launch that settled without readiness, so slow startups don't emit a
spurious launch_blocked; real onboarding screens still block.