feat(core): add agent protocol UI types and experimental flag

Also fixes a minor type error in nonInteractiveCli and mock ToolRegistry in tests that the new protocol exposes.

packages/cli/src/nonInteractiveCli.test.ts

@@ -1519,6 +1519,9 @@ describe('runNonInteractive', () => {
         name: 'ShellTool',
         description: 'A shell tool',
         run: vi.fn(),
+        build: vi.fn().mockReturnValue({
+          getDescription: () => 'A shell tool',
+        }),
       }),
       getFunctionDeclarations: vi.fn().mockReturnValue([{ name: 'ShellTool' }]),
     } as unknown as ToolRegistry);

packages/cli/src/nonInteractiveCli.ts

@@ -417,7 +417,9 @@ export async function runNonInteractive({
         return errToThrow;
       };
 
-      const runTerminalExitHandler = (handler: () => never): never => {
+      const runTerminalExitHandler = (
+        handler: () => void | never,
+      ): void | never => {
         terminalProcessExitHandled = true;
         return handler();
       };