fix: preserve BOM in text tool round-trips (#23797)

packages/opencode/src/util/bom.ts

@@ -0,0 +1,31 @@
+import { Effect } from "effect"
+import { AppFileSystem } from "@opencode-ai/shared/filesystem"
+
+const BOM_CODE = 0xfeff
+const BOM = String.fromCharCode(BOM_CODE)
+
+export function split(text: string) {
+  if (text.charCodeAt(0) !== BOM_CODE) return { bom: false, text }
+  return { bom: true, text: text.slice(1) }
+}
+
+export function join(text: string, bom: boolean) {
+  const stripped = split(text).text
+  if (!bom) return stripped
+  return BOM + stripped
+}
+
+export const readFile = Effect.fn("Bom.readFile")(function* (fs: AppFileSystem.Interface, filePath: string) {
+  return split(new TextDecoder("utf-8", { ignoreBOM: true }).decode(yield* fs.readFile(filePath)))
+})
+
+export const syncFile = Effect.fn("Bom.syncFile")(function* (
+  fs: AppFileSystem.Interface,
+  filePath: string,
+  bom: boolean,
+) {
+  const current = yield* readFile(fs, filePath)
+  if (current.bom === bom) return current.text
+  yield* fs.writeWithDirs(filePath, join(current.text, bom))
+  return current.text
+})