support custom tools (#2668)

.opencode/tool/foo.ts

@@ -0,0 +1,11 @@
+import z from "zod/v4"
+
+export default {
+  description: "foo tool for fooing",
+  args: {
+    foo: z.string().describe("foo"),
+  },
+  async execute() {
+    return "Hey fuck you!"
+  },
+}