fix(browse): restrictDirectoryPermissions warns and skips symlinked dirs

Closes the Windows Free Tests red: recent lane failures showed a
platform-unguarded POSIX mode-bit assertion ('Expected: 493' — a
symlink-skip test) from PR-branch variants; the KNOWN_WINDOWS_SAFE
force-include reason ('mode-bitmask hits are POSIX-branch only') did
not hold for that shape, and main had neither the guard nor the
behavior.

- product: lstat first; a symlinked dir gets a warning and a skip on
  both platforms — chmod AND icacls dereference the link, so
  restricting through a symlink hardens an unvetted target (and
  /inheritance:r could lock out its real owner). All callers already
  treat hardening as best-effort (try/catch).
- test: the symlink regression test, platform-aware — symlinkSync in
  the house try/catch skip pattern (Windows runners without Developer
  Mode can't create symlinks), mode-bit assertion guarded off win32,
  behavior assertions (no throw, warning text, target readable)
  everywhere; POSIX still proves the skip (0o755 unchanged, not 0o700)
- KNOWN_WINDOWS_SAFE reason updated to the now-true premise

20/20 pass on Linux.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/file-permissions.ts

@@ -140,8 +140,29 @@ export function restrictFilePermissions(filePath: string): void {
  * (CI = container inherit) inherit the single-user-full ACL — important
  * because child creations in `fs.writeFileSync(...)` without explicit
  * `restrictFilePermissions` still end up owner-only.
+ *
+ * Symlinked dirs are warned about and SKIPPED, never followed: both
+ * `chmod` and `icacls` dereference the link, so restricting through a
+ * symlink hardens whatever the link points at — a target the caller never
+ * vetted (and, with `/inheritance:r`, one we could lock its real owner out
+ * of). Skipping is best-effort-consistent with the rest of this module:
+ * the filesystem stays functional, we just don't hit the hardening target.
  */
 export function restrictDirectoryPermissions(dirPath: string): void {
+  try {
+    if (fs.lstatSync(dirPath).isSymbolicLink()) {
+      // biome-ignore lint/suspicious/noConsole: intentional user-facing warning
+      console.warn(
+        `[gstack] Refusing to restrict permissions through symlink ${dirPath} — skipping.\n` +
+        `  Restricting through a symlink would alter the link target instead. ` +
+        `Harden the real directory directly.`
+      );
+      return;
+    }
+  } catch {
+    // Path doesn't exist (or lstat failed) — fall through; both platform
+    // branches below already swallow failures on missing paths.
+  }
   if (process.platform === 'win32') {
     try {
       const user = currentUserPrincipal();

browse/test/file-permissions.test.ts

@@ -9,6 +9,11 @@
  *     we verify the helper doesn't throw and the file ends up accessible
  *     to the current user — the "doesn't crash, file still usable"
  *     contract the callers rely on.
+ *   - Every `mode & 0o777` bitmask assertion is platform-guarded: Windows
+ *     fakes POSIX mode bits (chmod is ~a no-op; dirs stat as 0o777), so a
+ *     bitmask expectation on win32 tests the runner, not our code. Symlink
+ *     fixtures are created in try/catch — Windows runners without Developer
+ *     Mode / admin can't create symlinks, and the test skips gracefully.
  */
 
 import { afterEach, beforeEach, describe, expect, test } from 'bun:test';
@@ -79,6 +84,47 @@ describe('restrictDirectoryPermissions', () => {
     expect(() => restrictDirectoryPermissions(d)).not.toThrow();
   });
 
+  test('warns and skips a symlinked dir without throwing', () => {
+    const real = path.join(tmpDir, 'real-target');
+    fs.mkdirSync(real);
+    if (process.platform !== 'win32') {
+      // chmod, not mkdir({ mode }), so a restrictive umask can't skew the
+      // starting bits we later assert were left untouched.
+      fs.chmodSync(real, 0o755);
+    }
+    const link = path.join(tmpDir, 'linked');
+    try {
+      fs.symlinkSync(real, link, 'dir');
+    } catch {
+      // Windows runners without Developer Mode / admin can't create
+      // symlinks (house pattern: security-audit-r2.test.ts skips the same
+      // way). Nothing to test without the link.
+      // biome-ignore lint/suspicious/noConsole: test-skip diagnostics
+      console.warn('Skipping: symlink creation failed (no symlink privilege)');
+      return;
+    }
+
+    const warnings: string[] = [];
+    const originalWarn = console.warn;
+    console.warn = (...args: unknown[]) => { warnings.push(args.map(String).join(' ')); };
+    try {
+      expect(() => restrictDirectoryPermissions(link)).not.toThrow();
+    } finally {
+      console.warn = originalWarn;
+    }
+    expect(warnings.some((w) => w.includes('symlink'))).toBe(true);
+
+    // The skip must leave the link target untouched. Mode bits are only
+    // meaningful on POSIX — Windows fakes stat().mode (dirs report 0o777
+    // no matter what), so asserting 0o755 there fails on runner semantics,
+    // not on our behavior. The no-throw + warn + still-usable checks are
+    // the meaningful win32 contract.
+    if (process.platform !== 'win32') {
+      expect(fs.statSync(real).mode & 0o777).toBe(0o755);
+    }
+    expect(() => fs.readdirSync(real)).not.toThrow();
+  });
+
   test('on Windows, the directory stays usable by the calling process', () => {
     if (process.platform !== 'win32') return;
     const d = path.join(tmpDir, 'still-usable');

scripts/test-free-shards.ts

@@ -282,12 +282,18 @@ const KNOWN_WINDOWS_SAFE: Array<{ file: string; reason: string }> = [
   {
     file: 'browse/test/file-permissions.test.ts',
     // Trips the POSIX-mode-bitmask pattern, but every `mode & 0o777` assertion
-    // is platform-guarded (win32 returns early / takes the icacls branch).
+    // is platform-guarded: win32-only tests return early, POSIX-only tests
+    // guard the bitmask behind `process.platform !== 'win32'`, and the
+    // symlink-skip regression test both wraps symlinkSync in try/catch
+    // (runners without Developer Mode can't create symlinks) and guards its
+    // bitmask — on win32 it asserts behavior (warns, skips, doesn't throw,
+    // target stays usable), never fake Windows mode bits (dirs stat 0o777
+    // there, so a 0o755 expectation fails on runner semantics, not our code).
     // This file carries the win32-only icacls-by-SID regression tests, which
     // can ONLY execute on windows-latest — excluding it here means the
     // machine-account ACL lockout regression is never exercised on the one
     // platform it bricks.
-    reason: 'mode-bitmask hits are POSIX-branch only; win32-only ACL regression tests must run on windows-latest',
+    reason: 'every mode-bitmask assertion is guarded off win32 (behavior asserted instead); win32-only ACL regression tests must run on windows-latest',
   },
   {
     file: 'browse/test/terminal-agent-owner-watchdog.test.ts',