fix(tui): enable tool access in fullscreen mode via NativeAgent

Fullscreen TUI was calling modelClient directly, bypassing the NativeAgent
tool loop entirely. Pass the agent through FullscreenTuiConfig → App and
use agent.process() for message handling, which enables the full tool
registry and executor.

src/frontends/tui/fullscreen.ts

@@ -4,6 +4,7 @@ import { App } from './components/index.js';
 import type { ManagedSession } from '../../session/index.js';
 import type { ModelClient } from '../../models/types.js';
 import type { ModelRouter } from '../../models/router.js';
+import type { NativeAgent } from '../../backends/native/agent.js';
 
 export interface FullscreenTuiConfig {
   session: ManagedSession;
@@ -11,6 +12,7 @@ export interface FullscreenTuiConfig {
   modelRouter?: ModelRouter;
   systemPrompt: string;
   model: string;
+  agent?: NativeAgent;
   onExit?: () => void;
 }
 
@@ -27,6 +29,7 @@ export async function startFullscreenTui(config: FullscreenTuiConfig): Promise<v
       modelRouter: config.modelRouter,
       systemPrompt: config.systemPrompt,
       model: config.model,
+      agent: config.agent,
       onExit: config.onExit,
     })
   );