effectify Plugin service (#18570)

packages/opencode/specs/effect-migration.md

@@ -75,6 +75,31 @@ export const ZodInfo = zod(Info) // derives z.ZodType from Schema.Union
 
 See `Auth.ZodInfo` for the canonical example.
 
+## InstanceState init patterns
+
+The `InstanceState.make` init callback receives a `Scope`, so you can use `Effect.acquireRelease`, `Effect.addFinalizer`, and `Effect.forkScoped` inside it. Resources acquired this way are automatically cleaned up when the instance is disposed or invalidated by `ScopedCache`. This makes it the right place for:
+
+- **Subscriptions**: Use `Effect.acquireRelease` to subscribe and auto-unsubscribe:
+
+```ts
+const cache = yield* InstanceState.make<State>(
+  Effect.fn("Foo.state")(function* (ctx) {
+    // ... load state ...
+
+    yield* Effect.acquireRelease(
+      Effect.sync(() => Bus.subscribeAll((event) => { /* handle */ })),
+      (unsub) => Effect.sync(unsub),
+    )
+
+    return { /* state */ }
+  }),
+)
+```
+
+- **Background fibers**: Use `Effect.forkScoped` — the fiber is interrupted on disposal.
+- **Side effects at init**: Config notification, event wiring, etc. all belong in the init closure. Callers just do `InstanceState.get(cache)` to trigger everything, and `ScopedCache` deduplicates automatically.
+
+The key insight: don't split init into a separate method with a `started` flag. Put everything in the `InstanceState.make` closure and let `ScopedCache` handle the run-once semantics.
 ## Scheduled Tasks
 
 For loops or periodic work, use `Effect.repeat` or `Effect.schedule` with `Effect.forkScoped` in the layer definition.
@@ -127,7 +152,7 @@ Fully migrated (single namespace, InstanceState where needed, flattened facade):
 
 Still open and likely worth migrating:
 
-- [ ] `Plugin`
+- [x] `Plugin`
 - [ ] `ToolRegistry`
 - [ ] `Pty`
 - [ ] `Worktree`