docs(doctor): document models.providers.api migration and stale-enum skip

Add the legacy `models.providers.*.api: "openai"` → `"openai-completions"`
migration to doctor's Current migrations list, and note the gateway startup
behavior that skips providers with future or unknown api enum values instead
of failing closed.

Traces to:
- 6a7980e984 fix(doctor): migrate legacy OpenAI provider api
- 147f4f50f5 fix(gateway): skip stale model provider api entries

docs/gateway/doctor.md

@@ -197,6 +197,7 @@ That stages grounded durable candidates into the short-term dreaming store while
     - `browser.ssrfPolicy.allowPrivateNetwork` → `browser.ssrfPolicy.dangerouslyAllowPrivateNetwork`
     - `browser.profiles.*.driver: "extension"` → `"existing-session"`
     - remove `browser.relayBindHost` (legacy extension relay setting)
+    - legacy `models.providers.*.api: "openai"` → `"openai-completions"` (gateway startup also skips providers whose `api` is set to a future or unknown enum value rather than failing closed)
 
     Doctor warnings also include account-default guidance for multi-account channels: