Speed up targeted opencode tests

Reduce avoidable setup costs in slow opencode tests while preserving reviewed coverage and recording the benchmark evidence for follow-up test-suite work.

packages/opencode/test/fixture/plugin.ts

@@ -0,0 +1,10 @@
+import { mkdir } from "fs/promises"
+import path from "path"
+
+export async function markPluginDependenciesReady(dir: string) {
+  await mkdir(path.join(dir, "node_modules"), { recursive: true })
+  await Bun.write(
+    path.join(dir, "package-lock.json"),
+    JSON.stringify({ packages: { "": { dependencies: { "@opencode-ai/plugin": "0.0.0" } } } }),
+  )
+}