fix(tui): reserve /runtime and block local tool-loop fallback

src/frontends/tui/minimal.test.ts

@@ -403,6 +403,29 @@ describe('MinimalTui backend command', () => {
     }
   });
 
+  it('prints guidance when /runtime is invoked in TUI mode', async () => {
+    const mockSession = {
+      id: 'test',
+      getHistory: () => [],
+      addMessage: vi.fn(),
+      clear: vi.fn(),
+      replaceHistory: vi.fn(),
+    };
+    const logSpy = vi.spyOn(console, 'log').mockImplementation(() => {});
+    try {
+      const tui = new MinimalTui({
+        session: asSession(mockSession),
+        modelClient: asModelClient({}),
+        systemPrompt: 'test',
+      });
+
+      await minimalTuiPrivates(tui).handleCommand({ type: 'runtime', input: 'status' });
+      expect(logSpy).toHaveBeenCalledWith(expect.stringContaining('Runtime backend mode command is not available in this TUI mode.'));
+    } finally {
+      logSpy.mockRestore();
+    }
+  });
+
   it('collects multiline input from /paste and sends as one message', async () => {
     const mockSession = {
       id: 'test',