fix: add missing clientId to github provider config for OAuth token refresh (#442)

The github provider in open-sse/config/providers.js was missing clientId,
causing refreshGitHubToken() to send client_id=undefined on 401 retry.
Also guard against undefined clientSecret in both refresh implementations.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

open-sse/config/providers.js

@@ -168,7 +168,8 @@ export const PROVIDERS = {
       "X-Initiator": "user",
       "Accept": "application/json",
       "Content-Type": "application/json"
-    }
+    },
+    clientId: "Iv1.b507a08c87ecfe98"
   },
   kiro: {
     baseUrl: "https://codewhisperer.us-east-1.amazonaws.com/generateAssistantResponse",