fix(fleet): contain credential trust roots

AMD1213-B4: reject symlinked auth ancestry and group/world-readable credential artifacts before composition can write.

packages/mosaic/src/commands/fleet-launch-command.spec.ts

@@ -1,4 +1,5 @@
 import {
+  chmodSync,
   lstatSync,
   mkdirSync,
   mkdtempSync,
@@ -319,7 +320,41 @@ describe('A3 credential validation', () => {
     ).toThrowError(/real, non-symlink credential file/);
   });
 
-  it('accepts a real credential file contained in the harness auth root', () => {
+  it('refuses an auth ancestor symlink that relocates the credential trust root', () => {
+    const fx = fixture();
+    rmSync(join(fx.userHome, 'auth'), { recursive: true, force: true });
+    const outsideAuth = join(fx.root, 'outside-auth');
+    const outsideBundle = join(outsideAuth, 'claude', 'fred_example.com');
+    mkdirSync(outsideBundle, { recursive: true });
+    writeFileSync(join(outsideBundle, '.credentials.json'), '{}\n', { mode: 0o600 });
+    writeFileSync(
+      join(outsideBundle, 'account.json'),
+      '{"oauthAccount":{"emailAddress":"[email protected]"}}\n',
+    );
+    symlinkSync('fred_example.com', join(outsideAuth, 'claude', 'primary'), 'dir');
+    symlinkSync(outsideAuth, join(fx.userHome, 'auth'), 'dir');
+
+    expect(() =>
+      resolveFleetLaunchComposition('fred', {
+        systemHome: fx.systemHome,
+        userHome: fx.userHome,
+      }),
+    ).toThrowError(/auth directory must be a real, non-symlink directory/);
+  });
+
+  it('refuses a group- or world-readable credential file', () => {
+    const fx = fixture();
+    chmodSync(join(fx.namedBundleDir, '.credentials.json'), 0o644);
+
+    expect(() =>
+      resolveFleetLaunchComposition('fred', {
+        systemHome: fx.systemHome,
+        userHome: fx.userHome,
+      }),
+    ).toThrowError(/credential file must not grant group or other permissions/);
+  });
+
+  it('accepts a real private credential file contained in the harness auth root', () => {
     const fx = fixture();
     const plan = resolveFleetLaunchComposition('fred', {
       systemHome: fx.systemHome,