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/src/core/custom-tools/types.ts

@@ -52,8 +52,8 @@ export interface CustomToolContext {
 /** Session event passed to onSession callback */
 export interface CustomToolSessionEvent {
 	/** Reason for the session event */
-	reason: "start" | "switch" | "branch" | "new" | "tree" | "shutdown";
-	/** Previous session file path, or undefined for "start", "new", and "shutdown" */
+	reason: "start" | "switch" | "branch" | "tree" | "shutdown";
+	/** Previous session file path, or undefined for "start" and "shutdown" */
 	previousSessionFile: string | undefined;
 }