feat: add experimental background subagents (#27084)

packages/opencode/test/session/prompt.test.ts

@@ -6,6 +6,7 @@ import path from "path"
 import { fileURLToPath, pathToFileURL } from "url"
 import { NamedError } from "@opencode-ai/core/util/error"
 import { Agent as AgentSvc } from "../../src/agent/agent"
+import { BackgroundJob } from "@/background/job"
 import { Bus } from "../../src/bus"
 import { Command } from "../../src/command"
 import { Config } from "@/config/config"
@@ -176,6 +177,7 @@ function makeHttp(input?: { processor?: "blocking" }) {
     lsp,
     mcp,
     AppFileSystem.defaultLayer,
+    BackgroundJob.defaultLayer,
     status,
     SyncEvent.defaultLayer,
   ).pipe(Layer.provideMerge(infra))