Add setEditorText/getEditorText to hook UI context, improve custom() API

- Add setEditorText() and getEditorText() to HookUIContext for prompt generator pattern
- custom() now accepts async factories for fire-and-forget work
- Add CancellableLoader component to tui package
- Add BorderedLoader component for hooks with cancel UI
- Export HookAPI, HookContext, HookFactory from main package
- Update all examples to import from packages instead of relative paths
- Update hooks.md and custom-tools.md documentation

fixes #350

packages/agent/README.md

@@ -298,6 +298,22 @@ const readFileTool: AgentTool = {
 agent.setTools([readFileTool]);
 ```
 
+### Error Handling
+
+**Throw an error** when a tool fails. Do not return error messages as content.
+
+```typescript
+execute: async (toolCallId, params, signal, onUpdate) => {
+  if (!fs.existsSync(params.path)) {
+    throw new Error(`File not found: ${params.path}`);
+  }
+  // Return content only on success
+  return { content: [{ type: "text", text: "..." }] };
+}
+```
+
+Thrown errors are caught by the agent and reported to the LLM as tool errors with `isError: true`.
+
 ## Proxy Usage
 
 For browser apps that proxy through a backend: