fix: use dependency injection for image-downloader fs mocks

The _fsForTest mutable object pattern fails on Linux Bun because
test and source files get different module instances. Switch to
passing fs operations as an optional parameter with defaults.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/github/utils/image-downloader.ts

@@ -2,11 +2,12 @@ import fs from "fs/promises";
 import path from "path";
 import type { Octokits } from "../api/client";
 
-// Mutable fs reference for cross-platform test compatibility.
-// On Linux, Bun gives test and source files different fs/promises objects,
-// so spyOn(fs, "mkdir") in a test file won't intercept calls here.
-// Tests override these methods directly instead.
-export const _fsForTest = {
+export type FsOps = {
+  mkdir: typeof fs.mkdir;
+  writeFile: typeof fs.writeFile;
+};
+
+const defaultFsOps: FsOps = {
   mkdir: fs.mkdir.bind(fs),
   writeFile: fs.writeFile.bind(fs),
 };
@@ -72,11 +73,12 @@ export async function downloadCommentImages(
   owner: string,
   repo: string,
   comments: CommentWithImages[],
+  fsOps: FsOps = defaultFsOps,
 ): Promise<Map<string, string>> {
   const urlToPathMap = new Map<string, string>();
   const downloadsDir = "/tmp/github-images";
 
-  await _fsForTest.mkdir(downloadsDir, { recursive: true });
+  await fsOps.mkdir(downloadsDir, { recursive: true });
 
   const { imageRegex, htmlImgRegex } = buildImageRegexes();
 
@@ -226,7 +228,7 @@ export async function downloadCommentImages(
           const arrayBuffer = await imageResponse.arrayBuffer();
           const buffer = Buffer.from(arrayBuffer);
 
-          await _fsForTest.writeFile(localPath, buffer);
+          await fsOps.writeFile(localPath, buffer);
           console.log(`✓ Saved: ${localPath}`);
 
           urlToPathMap.set(originalUrl, localPath);