refactor(hooks): split session events into individual typed events

Major changes:
- Replace monolithic SessionEvent with reason discriminator with individual
  event types: session_start, session_before_switch, session_switch,
  session_before_new, session_new, session_before_branch, session_branch,
  session_before_compact, session_compact, session_shutdown
- Each event has dedicated result type (SessionBeforeSwitchResult, etc.)
- HookHandler type now allows bare return statements (void in return type)
- HookAPI.on() has proper overloads for each event with correct typing

Additional fixes:
- AgentSession now always subscribes to agent in constructor (was only
  subscribing when external subscribe() called, breaking internal handlers)
- Standardize on undefined over null throughout codebase
- HookUIContext methods return undefined instead of null
- SessionManager methods return undefined instead of null
- Simplify hook exports to 'export type * from types.js'
- Add detailed JSDoc for skipConversationRestore vs cancel
- Fix createBranchedSession to rebuild index in persist mode
- newSession() now returns the session file path

Updated all example hooks, tests, and emission sites to use new event types.

packages/coding-agent/examples/hooks/file-trigger.ts

@@ -12,9 +12,7 @@ import * as fs from "node:fs";
 import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";
 
 export default function (pi: HookAPI) {
-	pi.on("session", async (event, ctx) => {
-		if (event.reason !== "start") return;
-
+	pi.on("session_start", async (_event, ctx) => {
 		const triggerFile = "/tmp/agent-trigger.txt";
 
 		fs.watch(triggerFile, () => {