feat(plugin): add promise plugin cleanup

packages/plugin/src/v2/promise/README.md

@@ -25,6 +25,15 @@ export default Plugin.define({
 ```
 
 Plugin setup registers hooks imperatively through each domain's `hook` method.
+It may return a synchronous or asynchronous cleanup function. OpenCode awaits
+the cleanup when the plugin is unloaded or replaced:
+
+```ts
+setup: async (ctx) => {
+  const timer = setInterval(refresh, 60_000)
+  return () => clearInterval(timer)
+}
+```
 
 Configuration supplied for the plugin is available as `ctx.options`.
 

packages/plugin/src/v2/promise/plugin.ts

@@ -26,9 +26,11 @@ export interface Context {
   readonly tool: ToolDomain
 }
 
+export type Cleanup = () => Promise<void> | void
+
 export interface Plugin {
   readonly id: string
-  readonly setup: (context: Context) => Promise<void> | void
+  readonly setup: (context: Context) => Promise<Cleanup | void> | Cleanup | void
 }
 
 export function define(plugin: Plugin) {