feat(cloud-agent): support codex cloud runs

[tatoalo]

Problem

cloud runs let users pickcodex via the UI but the selected runtime was not carried through the cloud-run path consistently

Changes

• added explicit cloud runtime fields for adapter, provider, model, and reasoning effort
• updated the cloud agent server startup path to select the configured adapter explicitly instead of relying on Claude-oriented defaults.
• added shared reasoning-effort capability helpers so local and cloud flows use the same adapter/model support rules

Companion: PostHog/posthog#54462

[jonathanlab]

LGTM, thank you!