support custom tools (#2668)

packages/plugin/src/example.ts

@@ -1,14 +1,20 @@
 import { Plugin } from "./index"
+import { tool } from "./tool"
 
-export const ExamplePlugin: Plugin = async ({
-  client: _client,
-  $: _shell,
-  project: _project,
-  directory: _directory,
-  worktree: _worktree,
-}) => {
+export const ExamplePlugin: Plugin = async (ctx) => {
   return {
     permission: {},
+    tool: {
+      mytool: tool((zod) => ({
+        description: "This is a custom tool tool",
+        args: {
+          foo: zod.string(),
+        },
+        async execute(args, ctx) {
+          return `Hello ${args.foo}!`
+        },
+      })),
+    },
     async "chat.params"(_input, output) {
       output.topP = 1
     },