docs: document Effect v4 fork API changes and InstanceState init patterns

Document the proper pattern for making service init() non-blocking by
forking at the call site rather than inside the InstanceState.make closure.
This prevents state initialization issues when multiple methods access
shared state during startup.

Add clarification that Effect.fork and Effect.forkDaemon are removed in
Effect v4 beta, with Effect.forkIn(scope) as the replacement for forking
fibers into specific scopes.

packages/opencode/AGENTS.md

@@ -39,6 +39,12 @@ See `specs/effect/migration.md` for the compact pattern reference and examples.
 - Do the work directly in the `InstanceState.make` closure — `ScopedCache` handles run-once semantics. Don't add fibers, `ensure()` callbacks, or `started` flags on top.
 - Use `Effect.addFinalizer` or `Effect.acquireRelease` inside the `InstanceState.make` closure for cleanup (subscriptions, process teardown, etc.).
 - Use `Effect.forkScoped` inside the closure for background stream consumers — the fiber is interrupted when the instance is disposed.
+- To make a service's `init()` non-blocking, fork `InstanceState.get(state)` at the `init()` call site (e.g. `Effect.forkIn(scope)`), not by forking work inside the `InstanceState.make` closure. Forking inside the closure leaves state incomplete for other methods that read it.
+- `src/project/bootstrap.ts` already wraps every service `init()` in `Effect.forkDetach`, so `init()` is fire-and-forget in production. Keep `init()` methods synchronous internally; the caller controls concurrency.
+
+## Effect v4 beta API
+
+- `Effect.fork` and `Effect.forkDaemon` do not exist. Use `Effect.forkIn(scope)` to fork a fiber into a specific scope.
 
 ## Preferred Effect services
 

packages/opencode/src/project/vcs.ts

@@ -1,4 +1,4 @@
-import { Effect, Layer, Context, Stream } from "effect"
+import { Effect, Layer, Context, Stream, Scope } from "effect"
 import { formatPatch, structuredPatch } from "diff"
 import path from "path"
 import { Bus } from "@/bus"
@@ -157,6 +157,7 @@ export const layer: Layer.Layer<Service, never, AppFileSystem.Service | Git.Serv
     const fs = yield* AppFileSystem.Service
     const git = yield* Git.Service
     const bus = yield* Bus.Service
+    const scope = yield* Scope.Scope
 
     const state = yield* InstanceState.make<State>(
       Effect.fn("Vcs.state")(function* (ctx) {
@@ -194,7 +195,7 @@ export const layer: Layer.Layer<Service, never, AppFileSystem.Service | Git.Serv
 
     return Service.of({
       init: Effect.fn("Vcs.init")(function* () {
-        yield* InstanceState.get(state)
+        yield* InstanceState.get(state).pipe(Effect.forkIn(scope))
       }),
       branch: Effect.fn("Vcs.branch")(function* () {
         return yield* InstanceState.use(state, (x) => x.current)