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/src/core/custom-tools/index.ts

@@ -0,0 +1,16 @@
+/**
+ * Custom tools module.
+ */
+
+export { discoverAndLoadCustomTools, loadCustomTools } from "./loader.js";
+export type {
+	CustomAgentTool,
+	CustomToolFactory,
+	CustomToolsLoadResult,
+	ExecResult,
+	LoadedCustomTool,
+	RenderResultOptions,
+	SessionEvent,
+	ToolAPI,
+	ToolUIContext,
+} from "./types.js";