feat(plugin): add promise plugin cleanup

packages/docs/build/plugins.mdx

@@ -148,7 +148,25 @@ export default Plugin.define({
 ```
 
 `setup` runs each time the plugin is activated. Register long-lived behavior
-during setup; do not wait there on an infinite event stream.
+during setup; do not wait there on an infinite event stream. It may return a
+synchronous or asynchronous cleanup function. OpenCode awaits that cleanup
+when the plugin is disabled, reloaded, or shut down:
+
+```ts
+setup: async (ctx) => {
+  const controller = new AbortController()
+  const task = synchronize(ctx, controller.signal)
+
+  return async () => {
+    controller.abort()
+    await task
+  }
+}
+```
+
+Hook registrations are released automatically with the same plugin scope. Use
+the returned cleanup for resources the plugin owns, such as timers, watchers,
+connections, and background tasks.
 
 ### Context
 
@@ -216,7 +234,8 @@ export default Plugin.define({
     }
 
     await refresh()
-    setInterval(() => void refresh().catch(console.error), 60_000)
+    const timer = setInterval(() => void refresh().catch(console.error), 60_000)
+    return () => clearInterval(timer)
   },
 })
 ```