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/test/agent-session-branching.test.ts

@@ -100,7 +100,7 @@ describe.skipIf(!API_KEY)("AgentSession branching", () => {
 		createSession(true);
 
 		// Verify sessions are disabled
-		expect(session.sessionFile).toBeNull();
+		expect(session.sessionFile).toBeUndefined();
 
 		// Send one message
 		await session.prompt("Say hi");
@@ -121,8 +121,8 @@ describe.skipIf(!API_KEY)("AgentSession branching", () => {
 		// After branching, conversation should be empty
 		expect(session.messages.length).toBe(0);
 
-		// Session file should still be null (no file created)
-		expect(session.sessionFile).toBeNull();
+		// Session file should still be undefined (no file created)
+		expect(session.sessionFile).toBeUndefined();
 	});
 
 	it("should branch from middle of conversation", async () => {