test: writing-style Codex fence expectation follows the plain start lookup (GST-882)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

test/writing-style-resolver.test.ts

@@ -94,7 +94,7 @@ describe('Writing Style preamble section', () => {
     const fenceLine = out.split('\n').find(l => l.includes('gstack-skill-start" --skill'));
     expect(fenceLine).toBeDefined();
     expect(fenceLine).not.toMatch(/~\/\.claude\//);
-    expect(fenceLine).toContain('$GSTACK_BIN');
+    expect(fenceLine).toContain('$GSTACK_ROOT/bin/');
   });
 
   test('tier 1 preamble does NOT include Writing Style section', () => {