fix: use dependency injection for image-downloader fs mocks

mock.module("fs/promises") on Linux Bun poisons the entire mock
tracking system — not just fs mocks, but ALL mock/spy assertions
(octokit mocks, spyOn(global, "fetch"), etc.) stop recording calls.

Replace with dependency injection: downloadCommentImages() now accepts
an optional fsDeps parameter. Tests pass mock mkdir/writeFile directly.
Production callers omit it and get real fs. No mock.module anywhere.

Fixes 18 test failures blocking CI on Forgejo Linux runners.

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

src/github/utils/image-downloader.ts

@@ -2,6 +2,15 @@ import fs from "fs/promises";
 import path from "path";
 import type { Octokits } from "../api/client";
 
+/** Injectable fs operations for cross-platform test compatibility.
+ *  On Linux Bun, mock.module("fs/promises") poisons the entire mock tracking
+ *  system, and spyOn(fs, "mkdir") gives test/source different object refs.
+ *  Dependency injection sidesteps all of this. */
+export interface FsDeps {
+  mkdir: typeof fs.mkdir;
+  writeFile: typeof fs.writeFile;
+}
+
 // 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
@@ -63,11 +72,13 @@ export async function downloadCommentImages(
   owner: string,
   repo: string,
   comments: CommentWithImages[],
+  fsDeps?: FsDeps,
 ): Promise<Map<string, string>> {
+  const { mkdir, writeFile } = fsDeps ?? fs;
   const urlToPathMap = new Map<string, string>();
   const downloadsDir = "/tmp/github-images";
 
-  await fs.mkdir(downloadsDir, { recursive: true });
+  await mkdir(downloadsDir, { recursive: true });
 
   const { imageRegex, htmlImgRegex } = buildImageRegexes();
 
@@ -217,7 +228,7 @@ export async function downloadCommentImages(
           const arrayBuffer = await imageResponse.arrayBuffer();
           const buffer = Buffer.from(arrayBuffer);
 
-          await fs.writeFile(localPath, buffer);
+          await writeFile(localPath, buffer);
           console.log(`✓ Saved: ${localPath}`);
 
           urlToPathMap.set(originalUrl, localPath);