Add a pre-flight version check that runs `codex --version` and blocks
session startup with a clear upgrade message when the installed CLI is
below v0.37.0. The check runs both synchronously at session start
(codexAppServerManager) and during provider health probes
(ProviderHealth), so users get fast feedback in the UI.

Introduces `codexCliVersion.ts` with semver parsing, comparison, and
formatting helpers shared by both call sites.