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/modes/interactive/components/model-selector.ts

@@ -36,18 +36,18 @@ export class ModelSelectorComponent extends Container {
 	private allModels: ModelItem[] = [];
 	private filteredModels: ModelItem[] = [];
 	private selectedIndex: number = 0;
-	private currentModel: Model<any> | null;
+	private currentModel?: Model<any>;
 	private settingsManager: SettingsManager;
 	private modelRegistry: ModelRegistry;
 	private onSelectCallback: (model: Model<any>) => void;
 	private onCancelCallback: () => void;
-	private errorMessage: string | null = null;
+	private errorMessage?: string;
 	private tui: TUI;
 	private scopedModels: ReadonlyArray<ScopedModelItem>;
 
 	constructor(
 		tui: TUI,
-		currentModel: Model<any> | null,
+		currentModel: Model<any> | undefined,
 		settingsManager: SettingsManager,
 		modelRegistry: ModelRegistry,
 		scopedModels: ReadonlyArray<ScopedModelItem>,