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/coding-agent/src/core/custom-tools/loader.ts

@@ -90,7 +90,9 @@ function createNoOpUIContext(): HookUIContext {
 		confirm: async () => false,
 		input: async () => undefined,
 		notify: () => {},
-		custom: () => ({ close: () => {}, requestRender: () => {} }),
+		custom: async () => undefined as never,
+		setEditorText: () => {},
+		getEditorText: () => "",
 	};
 }