* refactor(contracts): export ProviderStartOptions schema

* feat(contracts): add providerOptions to ThreadTurnStartCommand

* feat(web): send codex binary/home path settings to server

* feat(server): thread providerOptions through to session startup

* fix(contracts): define ProviderStartOptions inline to avoid circular dep

The import from provider.ts caused a ReferenceError at runtime because
of module initialization order in the bundled output.

* fix(server): preserve providerOptions across runtime mode changes and session recovery

Runtime mode toggles and server restarts silently dropped the custom
codex binary/home path, falling back to the default binary. Cache
providerOptions per thread in ProviderCommandReactor and persist it in
the session binding's runtimePayload so recovery can restore it.

Closes #486