Introduce a generic feature system where providers declare dynamic
features (toggles/selects) and the app renders controls automatically.
One message pair (set_agent_feature_request/response) handles all
feature mutations. Feature values persist and restore on agent resume.
First consumer: Codex fast mode (service_tier) — gated to supported
model families, with proper cleanup on model switch.
* feat: add live model and thinking option preferences UI
* refactor: remove agent variant support and clean up thinking options
* feat: live model + thinking presets (RPC)
