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/src/utils/tools-manager.ts

@@ -184,14 +184,14 @@ async function downloadTool(tool: "fd" | "rg"): Promise<string> {
 
 // Ensure a tool is available, downloading if necessary
 // Returns the path to the tool, or null if unavailable
-export async function ensureTool(tool: "fd" | "rg", silent: boolean = false): Promise<string | null> {
+export async function ensureTool(tool: "fd" | "rg", silent: boolean = false): Promise<string | undefined> {
 	const existingPath = getToolPath(tool);
 	if (existingPath) {
 		return existingPath;
 	}
 
 	const config = TOOLS[tool];
-	if (!config) return null;
+	if (!config) return undefined;
 
 	// Tool not found - download it
 	if (!silent) {
@@ -208,6 +208,6 @@ export async function ensureTool(tool: "fd" | "rg", silent: boolean = false): Pr
 		if (!silent) {
 			console.log(chalk.yellow(`Failed to download ${config.name}: ${e instanceof Error ? e.message : e}`));
 		}
-		return null;
+		return undefined;
 	}
 }