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/hello.ts

@@ -0,0 +1,20 @@
+import { Type } from "@sinclair/typebox";
+import type { CustomToolFactory } from "@mariozechner/pi-coding-agent";
+
+const factory: CustomToolFactory = (pi) => ({
+  name: "hello",
+  label: "Hello",
+  description: "A simple greeting tool",
+  parameters: Type.Object({
+    name: Type.String({ description: "Name to greet" }),
+  }),
+
+  async execute(toolCallId, params) {
+    return {
+      content: [{ type: "text", text: `Hello, ${params.name}!` }],
+      details: { greeted: params.name },
+    };
+  },
+});
+
+export default factory;