fix(opencode): migrate opencode.json to 'plugin' singular schema

The 'plugins' array-of-objects shape was the legacy schema; current
opencode-web3 requires 'plugin' as a flat array of paths/specs and
rejects the old shape with:

  Error: Configuration is invalid at packages/opencode/opencode.json
  Unrecognized key: 'plugins'

Migrate to the current schema so the plugin loads in fresh sessions.

Author: VoidChecksum

packages/opencode/opencode.json

@@ -1,9 +1,6 @@
 {
   "$schema": "https://opencode.ai/schemas/opencode.json",
-  "plugins": [
-    {
-      "name": "vigilo",
-      "module": "./dist/index.js"
-    }
+  "plugin": [
+    "./dist/index.js"
   ]
 }