fix: use mock.module for image-downloader tests

Replace the _fsForTest/DI patterns with Bun's native mock.module() API.
This mocks fs/promises at the module loader level, which works reliably
on Linux where jest.fn() call tracking fails cross-module. The source
code is now clean with no testing artifacts.

Also switches from jest.fn() to mock() (Bun's native API) for Octokit
mocks in these tests.

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

src/github/utils/image-downloader.ts

@@ -2,16 +2,6 @@ import fs from "fs/promises";
 import path from "path";
 import type { Octokits } from "../api/client";
 
-export type FsOps = {
-  mkdir: typeof fs.mkdir;
-  writeFile: typeof fs.writeFile;
-};
-
-const defaultFsOps: FsOps = {
-  mkdir: fs.mkdir.bind(fs),
-  writeFile: fs.writeFile.bind(fs),
-};
-
 // Build regexes at call time, reading GITHUB_SERVER_URL from process.env
 // directly. The config.ts export freezes the value at module load time,
 // which can be wrong when the Forgejo CI runner injects its own
@@ -73,12 +63,11 @@ 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 fsOps.mkdir(downloadsDir, { recursive: true });
+  await fs.mkdir(downloadsDir, { recursive: true });
 
   const { imageRegex, htmlImgRegex } = buildImageRegexes();
 
@@ -228,7 +217,7 @@ export async function downloadCommentImages(
           const arrayBuffer = await imageResponse.arrayBuffer();
           const buffer = Buffer.from(arrayBuffer);
 
-          await fsOps.writeFile(localPath, buffer);
+          await fs.writeFile(localPath, buffer);
           console.log(`✓ Saved: ${localPath}`);
 
           urlToPathMap.set(originalUrl, localPath);