chore: add opencode config for zen free model routing

Author: athal7

opencode.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "model": "opencode/gpt-5-nano",
+  "agent": {
+    "build": {
+      "model": "opencode/trinity-large-preview-free"
+    }
+  }
+}