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/coding-agent/examples/custom-tools/todo.ts

@@ -0,0 +1,192 @@
+/**
+ * Todo Tool - Demonstrates state management via session entries
+ *
+ * This tool stores state in tool result details (not external files),
+ * which allows proper branching - when you branch, the todo state
+ * is automatically correct for that point in history.
+ *
+ * The onSession callback reconstructs state by scanning past tool results.
+ */
+
+import { Type } from "@sinclair/typebox";
+import { StringEnum } from "@mariozechner/pi-ai";
+import { Text } from "@mariozechner/pi-tui";
+import type { CustomAgentTool, CustomToolFactory, ToolSessionEvent } from "@mariozechner/pi-coding-agent";
+
+interface Todo {
+	id: number;
+	text: string;
+	done: boolean;
+}
+
+// State stored in tool result details
+interface TodoDetails {
+	action: "list" | "add" | "toggle" | "clear";
+	todos: Todo[];
+	nextId: number;
+	error?: string;
+}
+
+// Define schema separately for proper type inference
+const TodoParams = Type.Object({
+	action: StringEnum(["list", "add", "toggle", "clear"] as const),
+	text: Type.Optional(Type.String({ description: "Todo text (for add)" })),
+	id: Type.Optional(Type.Number({ description: "Todo ID (for toggle)" })),
+});
+
+const factory: CustomToolFactory = (_pi) => {
+	// In-memory state (reconstructed from session on load)
+	let todos: Todo[] = [];
+	let nextId = 1;
+
+	/**
+	 * Reconstruct state from session entries.
+	 * Scans tool results for this tool and applies them in order.
+	 */
+	const reconstructState = (event: ToolSessionEvent) => {
+		todos = [];
+		nextId = 1;
+
+		for (const entry of event.entries) {
+			if (entry.type !== "message") continue;
+			const msg = entry.message;
+
+			// Tool results have role "toolResult"
+			if (msg.role !== "toolResult") continue;
+			if (msg.toolName !== "todo") continue;
+
+			const details = msg.details as TodoDetails | undefined;
+			if (details) {
+				todos = details.todos;
+				nextId = details.nextId;
+			}
+		}
+	};
+
+	const tool: CustomAgentTool<typeof TodoParams, TodoDetails> = {
+		name: "todo",
+		label: "Todo",
+		description: "Manage a todo list. Actions: list, add (text), toggle (id), clear",
+		parameters: TodoParams,
+
+		// Called on session start/switch/branch/clear
+		onSession: reconstructState,
+
+		async execute(_toolCallId, params) {
+			switch (params.action) {
+				case "list":
+					return {
+						content: [{ type: "text", text: todos.length ? todos.map((t) => `[${t.done ? "x" : " "}] #${t.id}: ${t.text}`).join("\n") : "No todos" }],
+						details: { action: "list", todos: [...todos], nextId },
+					};
+
+				case "add":
+					if (!params.text) {
+						return {
+							content: [{ type: "text", text: "Error: text required for add" }],
+							details: { action: "add", todos: [...todos], nextId, error: "text required" },
+						};
+					}
+					const newTodo: Todo = { id: nextId++, text: params.text, done: false };
+					todos.push(newTodo);
+					return {
+						content: [{ type: "text", text: `Added todo #${newTodo.id}: ${newTodo.text}` }],
+						details: { action: "add", todos: [...todos], nextId },
+					};
+
+				case "toggle":
+					if (params.id === undefined) {
+						return {
+							content: [{ type: "text", text: "Error: id required for toggle" }],
+							details: { action: "toggle", todos: [...todos], nextId, error: "id required" },
+						};
+					}
+					const todo = todos.find((t) => t.id === params.id);
+					if (!todo) {
+						return {
+							content: [{ type: "text", text: `Todo #${params.id} not found` }],
+							details: { action: "toggle", todos: [...todos], nextId, error: `#${params.id} not found` },
+						};
+					}
+					todo.done = !todo.done;
+					return {
+						content: [{ type: "text", text: `Todo #${todo.id} ${todo.done ? "completed" : "uncompleted"}` }],
+						details: { action: "toggle", todos: [...todos], nextId },
+					};
+
+				case "clear":
+					const count = todos.length;
+					todos = [];
+					nextId = 1;
+					return {
+						content: [{ type: "text", text: `Cleared ${count} todos` }],
+						details: { action: "clear", todos: [], nextId: 1 },
+					};
+
+				default:
+					return {
+						content: [{ type: "text", text: `Unknown action: ${params.action}` }],
+						details: { action: "list", todos: [...todos], nextId, error: `unknown action: ${params.action}` },
+					};
+			}
+		},
+
+		renderCall(args, theme) {
+			let text = theme.fg("toolTitle", theme.bold("todo ")) + theme.fg("muted", args.action);
+			if (args.text) text += " " + theme.fg("dim", `"${args.text}"`);
+			if (args.id !== undefined) text += " " + theme.fg("accent", `#${args.id}`);
+			return new Text(text, 0, 0);
+		},
+
+		renderResult(result, { expanded }, theme) {
+			const { details } = result;
+			if (!details) {
+				const text = result.content[0];
+				return new Text(text?.type === "text" ? text.text : "", 0, 0);
+			}
+
+			// Error
+			if (details.error) {
+				return new Text(theme.fg("error", `Error: ${details.error}`), 0, 0);
+			}
+
+			const todoList = details.todos;
+
+			switch (details.action) {
+				case "list":
+					if (todoList.length === 0) {
+						return new Text(theme.fg("dim", "No todos"), 0, 0);
+					}
+					let listText = theme.fg("muted", `${todoList.length} todo(s):`);
+					const display = expanded ? todoList : todoList.slice(0, 5);
+					for (const t of display) {
+						const check = t.done ? theme.fg("success", "✓") : theme.fg("dim", "○");
+						const itemText = t.done ? theme.fg("dim", t.text) : theme.fg("muted", t.text);
+						listText += "\n" + check + " " + theme.fg("accent", `#${t.id}`) + " " + itemText;
+					}
+					if (!expanded && todoList.length > 5) {
+						listText += "\n" + theme.fg("dim", `... ${todoList.length - 5} more`);
+					}
+					return new Text(listText, 0, 0);
+
+				case "add": {
+					const added = todoList[todoList.length - 1];
+					return new Text(theme.fg("success", "✓ Added ") + theme.fg("accent", `#${added.id}`) + " " + theme.fg("muted", added.text), 0, 0);
+				}
+
+				case "toggle": {
+					const text = result.content[0];
+					const msg = text?.type === "text" ? text.text : "";
+					return new Text(theme.fg("success", "✓ ") + theme.fg("muted", msg), 0, 0);
+				}
+
+				case "clear":
+					return new Text(theme.fg("success", "✓ ") + theme.fg("muted", "Cleared all todos"), 0, 0);
+			}
+		},
+	};
+
+	return tool;
+};
+
+export default factory;