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/custom-tools/loader.ts

@@ -86,9 +86,9 @@ function resolveToolPath(toolPath: string, cwd: string): string {
  */
 function createNoOpUIContext(): HookUIContext {
 	return {
-		select: async () => null,
+		select: async () => undefined,
 		confirm: async () => false,
-		input: async () => null,
+		input: async () => undefined,
 		notify: () => {},
 		custom: () => ({ close: () => {}, requestRender: () => {} }),
 	};

packages/coding-agent/src/core/custom-tools/types.ts

@@ -38,10 +38,10 @@ export interface ToolAPI {
 export interface SessionEvent {
 	/** All session entries (including pre-compaction history) */
 	entries: SessionEntry[];
-	/** Current session file path, or null in --no-session mode */
-	sessionFile: string | null;
-	/** Previous session file path, or null for "start" and "new" */
-	previousSessionFile: string | null;
+	/** Current session file path, or undefined in --no-session mode */
+	sessionFile: string | undefined;
+	/** Previous session file path, or undefined for "start" and "new" */
+	previousSessionFile: string | undefined;
 	/** Reason for the session event */
 	reason: "start" | "switch" | "branch" | "new";
 }