security: path traversal prevention for screenshot/pdf/eval (PR #26)

Add validateOutputPath() for screenshot/pdf/responsive (restricts to
/tmp and cwd) and validateReadPath() for eval (blocks .. sequences and
absolute paths outside safe dirs). 7 new tests.

Credit: Jah-yee (PR #26)

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

browse/src/meta-commands.ts

@@ -7,6 +7,18 @@ import { handleSnapshot } from './snapshot';
 import { getCleanText } from './read-commands';
 import * as Diff from 'diff';
 import * as fs from 'fs';
+import * as path from 'path';
+
+// Security: Path validation to prevent path traversal attacks
+const SAFE_DIRECTORIES = ['/tmp', process.cwd()];
+
+function validateOutputPath(filePath: string): void {
+  const resolved = path.resolve(filePath);
+  const isSafe = SAFE_DIRECTORIES.some(dir => resolved.startsWith(dir));
+  if (!isSafe) {
+    throw new Error(`Path must be within: ${SAFE_DIRECTORIES.join(', ')}`);
+  }
+}
 
 // Command sets for chain routing (mirrors server.ts — kept local to avoid circular import)
 const CHAIN_READ = new Set([
@@ -96,6 +108,7 @@ export async function handleMetaCommand(
     case 'screenshot': {
       const page = bm.getPage();
       const screenshotPath = args[0] || '/tmp/browse-screenshot.png';
+      validateOutputPath(screenshotPath);
       await page.screenshot({ path: screenshotPath, fullPage: true });
       return `Screenshot saved: ${screenshotPath}`;
     }
@@ -103,6 +116,7 @@ export async function handleMetaCommand(
     case 'pdf': {
       const page = bm.getPage();
       const pdfPath = args[0] || '/tmp/browse-page.pdf';
+      validateOutputPath(pdfPath);
       await page.pdf({ path: pdfPath, format: 'A4' });
       return `PDF saved: ${pdfPath}`;
     }
@@ -110,6 +124,7 @@ export async function handleMetaCommand(
     case 'responsive': {
       const page = bm.getPage();
       const prefix = args[0] || '/tmp/browse-responsive';
+      validateOutputPath(prefix);
       const viewports = [
         { name: 'mobile', width: 375, height: 812 },
         { name: 'tablet', width: 768, height: 1024 },