test(#2665): separator-safe containment in the regression assertion

startsWith(ambientConfigDir) also matches a sibling like
<tmp>/ambient-live-config-2, so it can report a leak that did not happen. Use
path.relative and check for '..' or an absolute result, the repo's usual shape.

The readdirSync assertion already carried the test, so this is cosmetic.

Addresses review finding: Nit 9.

tests/profile-output.test.cjs

@@ -223,8 +223,13 @@ describe('write-profile command', () => {
       [],
       'a call site that sandboxes HOME must not write into an ambient CLAUDE_CONFIG_DIR'
     );
+    // Containment via path.relative, not startsWith: startsWith(ambientConfigDir)
+    // also matches a SIBLING like `<tmp>/ambient-live-config-2`, so it can report
+    // a false leak. A path is inside the dir iff the relative path neither escapes
+    // with '..' nor is absolute.
+    const rel = path.relative(ambientConfigDir, out.profile_path);
     assert.ok(
-      !out.profile_path.startsWith(ambientConfigDir),
+      rel.startsWith('..') || path.isAbsolute(rel),
       `profile must not resolve under the ambient config dir, got: ${out.profile_path}`
     );
   });