chore(opencode): tune agent models and disable autoupdate

Update opencode.json to specify separate plan and build models,
enable GitLab feature flags, and turn off autoupdate to keep config
changes stable and predictable.

Author: Pedro Pombeiro

.config/opencode/opencode.json##class.Work

@@ -1,5 +1,13 @@
 {
   "$schema": "https://opencode.ai/config.json",
+  "agent": {
+    "plan": {
+      "model": "gitlab/duo-chat-opus-4-5"
+    },
+    "build": {
+      "model": "gitlab/duo-chat-sonnet-4-5"
+    }
+  },
   "default_agent": "plan",
   "plugin": [
     "opencode-gitlab-plugin@latest",
@@ -91,11 +99,18 @@
         "duo-chat-gpt-5-4-nano": {
           "cost": { "input": 0.50, "output": 2.00, "cache_read": 0.05 }
         }
+      },
+      "options": {
+        "instanceUrl": "https://gitlab.com",
+        "featureFlags": {
+          "duo_agent_platform_agentic_chat": true,
+          "duo_agent_platform": true
+        }
       }
     }
   },
   "share": "disabled",
-  "autoupdate": true,
+  "autoupdate": false,
   "server": {
     "hostname": "127.0.0.1",
     "mdns": false