fix(auq): harden error-fallback hook + harness per adversarial review

Codex pre-landing review found three real issues:
- The PostToolUse fallback hook shared source 'plan-tune-cathedral' with the
  question-log hook (same event+matcher); gstack-settings-hook replaces the entry,
  so it would have clobbered plan-tune capture. Give it its own 'auq-error-fallback'
  source (separate entry, both run); ALREADY_INSTALLED now requires both sources.
- isErrorResponse triggered on any string containing 'internal error'/'is_error',
  so a real answer or a {"is_error": false} payload could fire the fallback after a
  successful question. Narrow it to the missing-result sentinel + boolean is_error.
- The SDK runner mutated process.env.GSTACK_HEADLESS process-wide (leaked headless
  into later tests). Removed; GSTACK_HEADLESS=1 now lives in the eval package.json
  scripts, scoped to the invocation and inherited by the SDK child.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/auq-error-fallback-hook.test.ts

@@ -32,20 +32,29 @@ describe('isErrorResponse — only clear failures, never a real answer', () => {
     expect(isErrorResponse('[Tool result missing due to internal error]')).toBe(true);
   });
 
-  test('is_error / error object shapes are failures', () => {
+  test('is_error: true / error-field / sentinel-in-content are failures', () => {
     expect(isErrorResponse({ is_error: true })).toBe(true);
     expect(isErrorResponse({ isError: true })).toBe(true);
     expect(isErrorResponse({ error: 'boom' })).toBe(true);
-    expect(isErrorResponse({ content: '...internal error...' })).toBe(true);
+    expect(isErrorResponse({ content: 'Tool result missing due to internal error' })).toBe(true);
   });
 
   test('a real answer is NOT a failure (no false trigger)', () => {
     expect(isErrorResponse({ answers: [{ option_label: 'A' }] })).toBe(false);
     expect(isErrorResponse('A')).toBe(false);
-    // a choice that coincidentally contains the word "error" must not trip it
+    // a choice that coincidentally contains "error" must not trip it
     expect(isErrorResponse({ answers: [{ option_label: 'Fix the error' }] })).toBe(false);
     expect(isErrorResponse('Investigate the login error')).toBe(false);
   });
+
+  test('Codex review: narrow detection — generic "error"/"is_error" substrings do NOT trigger', () => {
+    // A real answer mentioning "internal error" must not be read as a failure.
+    expect(isErrorResponse('Investigate the internal error')).toBe(false);
+    // A serialized success payload containing the substring is_error:false must not trigger.
+    expect(isErrorResponse('{"is_error": false, "answer": "A"}')).toBe(false);
+    expect(isErrorResponse({ is_error: false })).toBe(false);
+    expect(isErrorResponse({ content: 'The page had an internal error we fixed' })).toBe(false);
+  });
 });
 
 describe('directiveFor — per-session-kind instruction', () => {