feat: per-CLI model picker for local agents (closes #8)

Each agent CLI declares its selectable models (and reasoning effort, for
Codex) on the daemon side; the frontend renders a model dropdown in the
avatar menu and the Settings dialog scoped to the currently picked CLI,
persists the choice per-agent in the AppConfig, and threads it through
/api/chat to the spawn argv. "Default" leaves the flag off so the CLI's
own config wins.

src/state/config.ts

@@ -11,6 +11,7 @@ export const DEFAULT_CONFIG: AppConfig = {
   skillId: null,
   designSystemId: null,
   onboardingCompleted: false,
+  agentModels: {},
 };
 
 export function loadConfig(): AppConfig {