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/core/auth-storage.ts

@@ -94,8 +94,8 @@ export class AuthStorage {
 	/**
 	 * Get credential for a provider.
 	 */
-	get(provider: string): AuthCredential | null {
-		return this.data[provider] ?? null;
+	get(provider: string): AuthCredential | undefined {
+		return this.data[provider] ?? undefined;
 	}
 
 	/**
@@ -191,7 +191,7 @@ export class AuthStorage {
 	 * 4. Environment variable
 	 * 5. Fallback resolver (models.json custom providers)
 	 */
-	async getApiKey(provider: string): Promise<string | null> {
+	async getApiKey(provider: string): Promise<string | undefined> {
 		// Runtime override takes highest priority
 		const runtimeKey = this.runtimeOverrides.get(provider);
 		if (runtimeKey) {
@@ -230,6 +230,6 @@ export class AuthStorage {
 		if (envKey) return envKey;
 
 		// Fall back to custom resolver (e.g., models.json custom providers)
-		return this.fallbackResolver?.(provider) ?? null;
+		return this.fallbackResolver?.(provider) ?? undefined;
 	}
 }