refactor(server): extract createApp function for server initialization

- Replace Server.App() with Server.Default() for internal server access
- Extract server app creation into Server.createApp(opts) for testability
- Move CORS whitelist from module-level variable to function parameter
- Update all tests to use Server.Default() instead of Server.App()

packages/opencode/src/cli/cmd/run.ts

@@ -667,7 +667,7 @@ export const RunCommand = cmd({
     await bootstrap(process.cwd(), async () => {
       const fetchFn = (async (input: RequestInfo | URL, init?: RequestInit) => {
         const request = new Request(input, init)
-        return Server.App().fetch(request)
+        return Server.Default().fetch(request)
       }) as typeof globalThis.fetch
       const sdk = createOpencodeClient({ baseUrl: "http://opencode.internal", fetch: fetchFn })
       await execute(sdk)