fix(sanitizer): strip alt text from reference-style markdown images (#1488)

stripMarkdownImageAltText removed alt text from inline images
(![alt](url)) but not reference-style images (![alt][ref]), because the
regex requires the "](" of the inline form. Alt text is a
hidden-instruction channel that reaches the prompt via sanitizeContent,
so the reference-style form let it survive.

Add a matching replace for the reference-style form (![alt][ref] ->
![][ref]), preserving the [ref] label so the image definition still
resolves. Adds regression tests.

Co-authored-by: Contributor <you@example.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/github/utils/sanitizer.ts

@@ -10,7 +10,13 @@ export function stripInvisibleCharacters(content: string): string {
 }
 
 export function stripMarkdownImageAltText(content: string): string {
-  return content.replace(/!\[[^\]]*\]\(/g, "![](");
+  // Inline images: ![alt](url) -> ![](url)
+  content = content.replace(/!\[[^\]]*\]\(/g, "![](");
+  // Reference-style images: ![alt][ref] -> ![][ref] (keep the label, drop the
+  // alt text, which is otherwise a hidden-instruction channel just like the
+  // inline form above).
+  content = content.replace(/!\[[^\]]*\](\[[^\]]*\])/g, "![]$1");
+  return content;
 }
 
 export function stripMarkdownLinkTitles(content: string): string {

test/sanitizer.test.ts

@@ -59,6 +59,22 @@ describe("stripMarkdownImageAltText", () => {
   it("should handle empty alt text", () => {
     expect(stripMarkdownImageAltText("![](image.png)")).toBe("![](image.png)");
   });
+
+  it("should remove alt text from reference-style images", () => {
+    expect(stripMarkdownImageAltText("![example alt text][img1]")).toBe(
+      "![][img1]",
+    );
+    expect(
+      stripMarkdownImageAltText("Text ![description][ref] more text"),
+    ).toBe("Text ![][ref] more text");
+  });
+
+  it("should preserve the reference label of a reference-style image", () => {
+    // the [ref] label must survive so the image definition still resolves;
+    // only the alt text (the injection channel) is removed
+    expect(stripMarkdownImageAltText("![alt][my-ref]")).toBe("![][my-ref]");
+    expect(stripMarkdownImageAltText("![][keep]")).toBe("![][keep]");
+  });
 });
 
 describe("stripMarkdownLinkTitles", () => {