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/tui/src/index.ts

@@ -9,6 +9,7 @@ export {
 } from "./autocomplete.js";
 // Components
 export { Box } from "./components/box.js";
+export { CancellableLoader } from "./components/cancellable-loader.js";
 export { Editor, type EditorTheme } from "./components/editor.js";
 export { Image, type ImageOptions, type ImageTheme } from "./components/image.js";
 export { Input } from "./components/input.js";