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/print-mode.ts

@@ -45,10 +45,9 @@ export async function runPrintMode(
 		hookRunner.setAppendEntryHandler((customType, data) => {
 			session.sessionManager.appendCustomEntry(customType, data);
 		});
-		// Emit session event
+		// Emit session_start event
 		await hookRunner.emit({
-			type: "session",
-			reason: "start",
+			type: "session_start",
 		});
 	}
 
@@ -59,7 +58,7 @@ export async function runPrintMode(
 				await tool.onSession({
 					entries,
 					sessionFile: session.sessionFile,
-					previousSessionFile: null,
+					previousSessionFile: undefined,
 					reason: "start",
 				});
 			} catch (_err) {