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/hooks/git-checkpoint.ts

@@ -0,0 +1,48 @@
+/**
+ * Git Checkpoint Hook
+ *
+ * Creates git stash checkpoints at each turn so /branch can restore code state.
+ * When branching, offers to restore code to that point in history.
+ */
+
+import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";
+
+export default function (pi: HookAPI) {
+	const checkpoints = new Map<number, string>();
+
+	pi.on("turn_start", async (event, ctx) => {
+		// Create a git stash entry before LLM makes changes
+		const { stdout } = await ctx.exec("git", ["stash", "create"]);
+		const ref = stdout.trim();
+		if (ref) {
+			checkpoints.set(event.turnIndex, ref);
+		}
+	});
+
+	pi.on("branch", async (event, ctx) => {
+		const ref = checkpoints.get(event.targetTurnIndex);
+		if (!ref) return undefined;
+
+		if (!ctx.hasUI) {
+			// In non-interactive mode, don't restore automatically
+			return undefined;
+		}
+
+		const choice = await ctx.ui.select("Restore code state?", [
+			"Yes, restore code to that point",
+			"No, keep current code",
+		]);
+
+		if (choice?.startsWith("Yes")) {
+			await ctx.exec("git", ["stash", "apply", ref]);
+			ctx.ui.notify("Code restored to checkpoint", "info");
+		}
+
+		return undefined;
+	});
+
+	pi.on("agent_end", async () => {
+		// Clear checkpoints after agent completes
+		checkpoints.clear();
+	});
+}