fix(coding-agent): sanitize markdown links in exported session HTML (#3532)

marked v15 does not filter dangerous URL protocols. The default link
renderer passes href values through verbatim, so markdown like
`[click](javascript:alert(1))` renders as a clickable XSS link in
shared/exported session HTML.

Add custom link and image renderers that:
- Block javascript:, vbscript:, and data: protocol URLs
- Escape href/title/alt attributes via escapeHtml()

Also escape img.mimeType in session image rendering to prevent
attribute breakout from crafted session JSONL.

Fixes #3531

packages/coding-agent/test/export-html-xss.test.ts

@@ -0,0 +1,28 @@
+import { readFileSync } from "fs";
+import { describe, expect, it } from "vitest";
+
+describe("export HTML markdown link sanitization", () => {
+	const templateJs = readFileSync(new URL("../src/core/export-html/template.js", import.meta.url), "utf-8");
+
+	it("overrides the marked link renderer to block javascript: protocol", () => {
+		// The custom link renderer must check for dangerous protocols
+		expect(templateJs).toMatch(/link\s*\(\s*token\s*\)/);
+		expect(templateJs).toMatch(/javascript/i);
+		expect(templateJs).toMatch(/vbscript/i);
+	});
+
+	it("overrides the marked image renderer to block javascript: protocol", () => {
+		expect(templateJs).toMatch(/image\s*\(\s*token\s*\)/);
+	});
+
+	it("escapes href attributes in the custom link renderer", () => {
+		// The link renderer must escape href values to prevent attribute breakout
+		expect(templateJs).toMatch(/escapeHtml\(href\)/);
+	});
+
+	it("escapes image mimeType attributes", () => {
+		// Image mimeType must be escaped to prevent attribute breakout
+		expect(templateJs).not.toMatch(/\$\{img\.mimeType\}/);
+		expect(templateJs).toMatch(/escapeHtml\(img\.mimeType/);
+	});
+});