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/plugin/index.ts

@@ -25,8 +25,7 @@ export namespace Plugin {
     const client = createOpencodeClient({
       baseUrl: "http://localhost:4096",
       directory: Instance.directory,
-      // @ts-ignore - fetch type incompatibility
-      fetch: async (...args) => Server.App().fetch(...args),
+      fetch: async (...args) => Server.Default().fetch(...args),
     })
     const config = await Config.get()
     const hooks: Hooks[] = []
@@ -35,7 +34,9 @@ export namespace Plugin {
       project: Instance.project,
       worktree: Instance.worktree,
       directory: Instance.directory,
-      serverUrl: Server.url(),
+      get serverUrl(): URL {
+        throw new Error("Server URL is no longer supported in plugins")
+      },
       $: Bun.$,
     }