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>

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", () => {