fix(browse): add path validation to upload command (#821)

Add isPathWithin() and path traversal checks to the upload command,
blocking file exfiltration via crafted upload paths. Uses existing
SAFE_DIRECTORIES constant instead of a local copy. Adds 3 regression tests.

Co-authored-by: garagon <garagon@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/test/path-validation.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect } from 'bun:test';
 import { validateOutputPath } from '../src/meta-commands';
 import { validateReadPath } from '../src/read-commands';
-import { symlinkSync, unlinkSync, writeFileSync } from 'fs';
+import { readFileSync, symlinkSync, unlinkSync, writeFileSync } from 'fs';
 import { tmpdir } from 'os';
 import { join } from 'path';
 
@@ -35,6 +35,26 @@ describe('validateOutputPath', () => {
   });
 });
 
+describe('upload command path validation', () => {
+  const src = readFileSync(join(__dirname, '..', 'src', 'write-commands.ts'), 'utf-8');
+
+  it('validates upload paths with isPathWithin', () => {
+    const uploadBlock = src.slice(src.indexOf("case 'upload'"), src.indexOf("case 'dialog-accept'"));
+    expect(uploadBlock).toContain('isPathWithin');
+  });
+
+  it('blocks path traversal in upload', () => {
+    const uploadBlock = src.slice(src.indexOf("case 'upload'"), src.indexOf("case 'dialog-accept'"));
+    expect(uploadBlock).toContain("'..'");
+  });
+
+  it('checks absolute paths against safe directories', () => {
+    const uploadBlock = src.slice(src.indexOf("case 'upload'"), src.indexOf("case 'dialog-accept'"));
+    expect(uploadBlock).toContain('path.isAbsolute');
+    expect(uploadBlock).toContain('SAFE_DIRECTORIES');
+  });
+});
+
 describe('validateReadPath', () => {
   it('allows absolute paths within /tmp', () => {
     expect(() => validateReadPath('/tmp/script.js')).not.toThrow();