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/session-manager/tree-traversal.test.ts

@@ -451,7 +451,7 @@ describe("createBranchedSession", () => {
 
 		// Create branched session from id2 (should only have 1 -> 2)
 		const result = session.createBranchedSession(id2);
-		expect(result).toBeNull(); // in-memory returns null
+		expect(result).toBeUndefined(); // in-memory returns null
 
 		// Session should now only have entries 1 and 2
 		const entries = session.getEntries();