fix: prevent zip slip attacks in DXT unpack function (#74)

* fix: prevent zip slip attacks in DXT unpack function

Add path validation to prevent directory traversal attacks when unpacking DXT files.
The fix validates that extracted file paths remain within the intended output directory
before writing files to disk.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* fix: make zip slip protection cross-platform compatible

Use path.sep instead of hardcoded '/' separator to ensure path validation
works correctly on both Windows and Unix systems. This fixes failing tests
on Windows while maintaining security protection against directory traversal.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/cli/unpack.ts

@@ -1,6 +1,6 @@
 import { unzipSync } from "fflate";
 import { chmodSync, existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
-import { join, resolve } from "path";
+import { join, resolve, sep } from "path";
 
 import { extractSignatureBlock } from "../node/sign.js";
 import { getLogger } from "../shared/log.js";
@@ -85,6 +85,17 @@ export async function unpackExtension({
       if (Object.prototype.hasOwnProperty.call(decompressed, relativePath)) {
         const data = decompressed[relativePath];
         const fullPath = join(finalOutputDir, relativePath);
+
+        // Prevent zip slip attacks by validating the resolved path
+        const normalizedPath = resolve(fullPath);
+        const normalizedOutputDir = resolve(finalOutputDir);
+        if (
+          !normalizedPath.startsWith(normalizedOutputDir + sep) &&
+          normalizedPath !== normalizedOutputDir
+        ) {
+          throw new Error(`Path traversal attempt detected: ${relativePath}`);
+        }
+
         const dir = join(fullPath, "..");
         if (!existsSync(dir)) {
           mkdirSync(dir, { recursive: true });