Consolidate session events: remove session_before_new/session_new, add reason field to switch events

- Remove session_before_new and session_new hook events
- Add reason: 'new' | 'resume' to session_before_switch and session_switch events
- Remove 'new' reason from custom tool onSession (use 'switch' for both /new and /resume)
- Rename reset() to newSession(options?) in AgentSession
- Add NewSessionOptions with optional parentSession for lineage tracking
- Rename branchedFrom to parentSession in SessionHeader
- Rename RPC reset command to new_session with optional parentSession
- Update example hooks to use new event structure
- Update documentation and changelog

Based on discussion in #293

packages/coding-agent/docs/sdk.md

@@ -100,7 +100,7 @@ interface AgentSession {
   isStreaming: boolean;
   
   // Session management
-  reset(): Promise<boolean>;  // Returns false if cancelled by hook
+  newSession(options?: { parentSession?: string }): Promise<boolean>;  // Returns false if cancelled by hook
   switchSession(sessionPath: string): Promise<boolean>;
   
   // Branching