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/index.ts

@@ -38,33 +38,7 @@ export type {
 	ToolUIContext,
 } from "./core/custom-tools/index.js";
 export { discoverAndLoadCustomTools, loadCustomTools } from "./core/custom-tools/index.js";
-export type {
-	AgentEndEvent,
-	AgentStartEvent,
-	BashToolResultEvent,
-	BeforeAgentStartEvent,
-	BeforeAgentStartEventResult,
-	CustomToolResultEvent,
-	EditToolResultEvent,
-	FindToolResultEvent,
-	GrepToolResultEvent,
-	HookAPI,
-	HookEvent,
-	HookEventContext,
-	HookFactory,
-	HookUIContext,
-	LsToolResultEvent,
-	ReadToolResultEvent,
-	SessionEvent,
-	SessionEventResult,
-	ToolCallEvent,
-	ToolCallEventResult,
-	ToolResultEvent,
-	ToolResultEventResult,
-	TurnEndEvent,
-	TurnStartEvent,
-	WriteToolResultEvent,
-} from "./core/hooks/index.js";
+export type * from "./core/hooks/index.js";
 // Hook system types and type guards
 export {
 	isBashToolResult,