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/components/cancellable-loader.ts

@@ -0,0 +1,39 @@
+import { isEscape } from "../keys.js";
+import { Loader } from "./loader.js";
+
+/**
+ * Loader that can be cancelled with Escape.
+ * Extends Loader with an AbortSignal for cancelling async operations.
+ *
+ * @example
+ * const loader = new CancellableLoader(tui, cyan, dim, "Working...");
+ * loader.onAbort = () => done(null);
+ * doWork(loader.signal).then(done);
+ */
+export class CancellableLoader extends Loader {
+	private abortController = new AbortController();
+
+	/** Called when user presses Escape */
+	onAbort?: () => void;
+
+	/** AbortSignal that is aborted when user presses Escape */
+	get signal(): AbortSignal {
+		return this.abortController.signal;
+	}
+
+	/** Whether the loader was aborted */
+	get aborted(): boolean {
+		return this.abortController.signal.aborted;
+	}
+
+	handleInput(data: string): void {
+		if (isEscape(data)) {
+			this.abortController.abort();
+			this.onAbort?.();
+		}
+	}
+
+	dispose(): void {
+		this.stop();
+	}
+}