core: return structured server info with stop method from workspace server

- Enables graceful server shutdown for workspace management
- Removes unsupported serverUrl getter that threw errors in plugin context

packages/opencode/src/plugin/index.ts

@@ -34,10 +34,6 @@ export namespace Plugin {
       project: Instance.project,
       worktree: Instance.worktree,
       directory: Instance.directory,
-      get serverUrl(): URL {
-        throw new Error("Server URL is no longer supported in plugins")
-      },
-      $: () => {},
     }
 
     for (const plugin of INTERNAL_PLUGINS) {