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/bash-executor.ts

@@ -29,8 +29,8 @@ export interface BashExecutorOptions {
 export interface BashResult {
 	/** Combined stdout + stderr output (sanitized, possibly truncated) */
 	output: string;
-	/** Process exit code (null if killed/cancelled) */
-	exitCode: number | null;
+	/** Process exit code (undefined if killed/cancelled) */
+	exitCode: number | undefined;
 	/** Whether the command was cancelled via signal */
 	cancelled: boolean;
 	/** Whether the output was truncated */
@@ -88,7 +88,7 @@ export function executeBash(command: string, options?: BashExecutorOptions): Pro
 				child.kill();
 				resolve({
 					output: "",
-					exitCode: null,
+					exitCode: undefined,
 					cancelled: true,
 					truncated: false,
 				});
@@ -154,7 +154,7 @@ export function executeBash(command: string, options?: BashExecutorOptions): Pro
 
 			resolve({
 				output: truncationResult.truncated ? truncationResult.content : fullOutput,
-				exitCode: code,
+				exitCode: cancelled ? undefined : code,
 				cancelled,
 				truncated: truncationResult.truncated,
 				fullOutputPath: tempFilePath,