refactor(open-sse): source mimo-free/opencode base URL from PROVIDERS

CHAT_URL and opencode buildUrl base now read from PROVIDERS instead of repeating
the literal. Values identical; providers byte-for-byte + gate clean.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/executors/opencode.js

@@ -15,7 +15,7 @@ export class OpenCodeExecutor extends BaseExecutor {
   }
 
   buildUrl(model) {
-    const base = "https://opencode.ai";
+    const base = this.config.baseUrl;
     return MESSAGES_MODELS.has(model)
       ? `${base}/zen/v1/messages`
       : `${base}/zen/v1/chat/completions`;