fix: proper gstack description in openai.yaml + block Codex from rewriting it

Codex kept overwriting agents/openai.yaml with a browse-only description.
Two fixes: (1) better description covering full PM/dev/eng/CEO/QA scope,
(2) add agents/ to the filesystem boundary so Codex stops modifying it.

test/gen-skill-docs.test.ts

@@ -1304,6 +1304,7 @@ describe('Codex generation (--host codex)', () => {
     const content = fs.readFileSync(rootMetadata, 'utf-8');
     expect(content).toContain('display_name: "gstack"');
     expect(content).toContain('Use $gstack to locate the bundled gstack skills.');
+    expect(content).toContain('allow_implicit_invocation: true');
   });
 
   test('codexSkillName mapping: root is gstack, others are gstack-{dir}', () => {