Use external wrapper for zod import

Import zod from './external/zod' wrapper instead of directly from 'zod'.
This follows the same pattern as other external dependencies like cacache.

The external/zod.js wrapper uses CommonJS module.exports pattern which:
- Works correctly in regular Node.js environment
- Gets bundled by build-externals.mjs for production
- Still has resolution issues in vitest test environment

Note: ipc.test.ts still fails due to vitest/CommonJS interop issue.

Author: jdalton

src/external/zod.js

@@ -1,2 +1 @@
-export { z } from 'zod'
-export * from 'zod'
+module.exports = require('zod')

src/ipc.ts

@@ -33,7 +33,7 @@ import { promises as fs } from 'node:fs'
 import os from 'node:os'
 import path from 'node:path'
 
-import { z } from 'zod'
+import { z } from './external/zod'
 
 // Define BufferEncoding type for TypeScript compatibility.
 type BufferEncoding = globalThis.BufferEncoding