feat: add provider-declared features system with Codex fast mode (#186)

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.

packages/app/src/stores/session-store.ts

@@ -10,6 +10,7 @@ import type {
   AgentPermissionResponse,
   AgentPermissionRequest,
   AgentSessionConfig,
+  AgentFeature,
   AgentProvider,
   AgentMode,
   AgentCapabilityFlags,
@@ -99,6 +100,7 @@ export interface Agent {
   title: string | null;
   cwd: string;
   model: string | null;
+  features?: AgentFeature[];
   thinkingOptionId?: string | null;
   requiresAttention?: boolean;
   attentionReason?: "finished" | "error" | "permission" | null;