Custom tools with session lifecycle, examples for hooks and tools

- Custom tools: TypeScript modules that extend pi with new tools
  - Custom TUI rendering via renderCall/renderResult
  - User interaction via pi.ui (select, confirm, input, notify)
  - Session lifecycle via onSession callback for state reconstruction
  - Examples: todo.ts, question.ts, hello.ts

- Hook examples: permission-gate, git-checkpoint, protected-paths

- Session lifecycle centralized in AgentSession
  - Works across all modes (interactive, print, RPC)
  - Unified session event for hooks (replaces session_start/session_switch)

- Box component added to pi-tui

- Examples bundled in npm and binary releases

Fixes #190

packages/tui/src/components/box.ts

@@ -0,0 +1,96 @@
+import type { Component } from "../tui.js";
+import { applyBackgroundToLine, visibleWidth } from "../utils.js";
+
+/**
+ * Box component - a container that applies padding and background to all children
+ */
+export class Box implements Component {
+	children: Component[] = [];
+	private paddingX: number;
+	private paddingY: number;
+	private bgFn?: (text: string) => string;
+
+	constructor(paddingX = 1, paddingY = 1, bgFn?: (text: string) => string) {
+		this.paddingX = paddingX;
+		this.paddingY = paddingY;
+		this.bgFn = bgFn;
+	}
+
+	addChild(component: Component): void {
+		this.children.push(component);
+	}
+
+	removeChild(component: Component): void {
+		const index = this.children.indexOf(component);
+		if (index !== -1) {
+			this.children.splice(index, 1);
+		}
+	}
+
+	clear(): void {
+		this.children = [];
+	}
+
+	setBgFn(bgFn?: (text: string) => string): void {
+		this.bgFn = bgFn;
+	}
+
+	invalidate(): void {
+		for (const child of this.children) {
+			child.invalidate?.();
+		}
+	}
+
+	render(width: number): string[] {
+		if (this.children.length === 0) {
+			return [];
+		}
+
+		const contentWidth = Math.max(1, width - this.paddingX * 2);
+		const leftPad = " ".repeat(this.paddingX);
+
+		// Render all children
+		const childLines: string[] = [];
+		for (const child of this.children) {
+			const lines = child.render(contentWidth);
+			for (const line of lines) {
+				childLines.push(leftPad + line);
+			}
+		}
+
+		if (childLines.length === 0) {
+			return [];
+		}
+
+		// Apply background and padding
+		const result: string[] = [];
+
+		// Top padding
+		for (let i = 0; i < this.paddingY; i++) {
+			result.push(this.applyBg("", width));
+		}
+
+		// Content
+		for (const line of childLines) {
+			result.push(this.applyBg(line, width));
+		}
+
+		// Bottom padding
+		for (let i = 0; i < this.paddingY; i++) {
+			result.push(this.applyBg("", width));
+		}
+
+		return result;
+	}
+
+	private applyBg(line: string, width: number): string {
+		const visLen = visibleWidth(line);
+		const padNeeded = Math.max(0, width - visLen);
+		const padded = line + " ".repeat(padNeeded);
+
+		if (this.bgFn) {
+			return applyBackgroundToLine(padded, width, this.bgFn);
+		}
+		return padded;
+	}
+}