fix(setup): stop presenting ChatGPT OAuth as the default voice fallback

Doctor and dev-fresh told every user without an OpenAI key that voice could
use ChatGPT OAuth, whether or not Codex was installed. Say voice is off
until a key is added or a Codex sign-in is selected, and mark that path
experimental.

src/setupDoctor.test.mjs

@@ -279,7 +279,7 @@ test('doctor describes the credential ladder without exposing values', () => {
       ...credentials,
       OPENAI_API_KEY: { configured: false },
     }).voice,
-    'ChatGPT/Codex OAuth can be selected at runtime',
+    'off until an OpenAI key is added, or a Codex ChatGPT sign-in is selected in Provider Settings (experimental)',
   );
   assert.match(capabilities.missions, /token allowance/);
   assert.equal(