Add packages/opencode-config and use it

packages/opencode-config/opencode.jsonc

@@ -0,0 +1,3 @@
+{
+  "$schema": "https://opencode.ai/config.json"
+}

packages/opencode-config/plugin/hello.js

@@ -0,0 +1,18 @@
+import { tool } from "@opencode-ai/plugin/tool"
+
+export async function HelloPlugin() {
+  return {
+    tool: {
+      hello: tool({
+        description: "Return a friendly greeting",
+        args: {
+          name: tool.schema.string().optional().describe("Name to greet"),
+        },
+        async execute(args) {
+          const target = args.name?.trim() || "CodeNomad"
+          return `Hello, ${target}!`
+        },
+      }),
+    },
+  }
+}