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/modes/interactive/components/bordered-loader.ts

@@ -0,0 +1,41 @@
+import { CancellableLoader, Container, Spacer, Text, type TUI } from "@mariozechner/pi-tui";
+import type { Theme } from "../theme/theme.js";
+import { DynamicBorder } from "./dynamic-border.js";
+
+/** Loader wrapped with borders for hook UI */
+export class BorderedLoader extends Container {
+	private loader: CancellableLoader;
+
+	constructor(tui: TUI, theme: Theme, message: string) {
+		super();
+		this.addChild(new DynamicBorder());
+		this.addChild(new Spacer(1));
+		this.loader = new CancellableLoader(
+			tui,
+			(s) => theme.fg("accent", s),
+			(s) => theme.fg("muted", s),
+			message,
+		);
+		this.addChild(this.loader);
+		this.addChild(new Spacer(1));
+		this.addChild(new Text(theme.fg("muted", "esc cancel"), 1, 0));
+		this.addChild(new Spacer(1));
+		this.addChild(new DynamicBorder());
+	}
+
+	get signal(): AbortSignal {
+		return this.loader.signal;
+	}
+
+	set onAbort(fn: (() => void) | undefined) {
+		this.loader.onAbort = fn;
+	}
+
+	handleInput(data: string): void {
+		this.loader.handleInput(data);
+	}
+
+	dispose(): void {
+		this.loader.dispose();
+	}
+}