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/modes/rpc/rpc-client.ts

@@ -187,11 +187,12 @@ export class RpcClient {
 	}
 
 	/**
-	 * Reset session (clear all messages).
-	 * @returns Object with `cancelled: true` if a hook cancelled the reset
+	 * Start a new session, optionally with parent tracking.
+	 * @param parentSession - Optional parent session path for lineage tracking
+	 * @returns Object with `cancelled: true` if a hook cancelled the new session
 	 */
-	async reset(): Promise<{ cancelled: boolean }> {
-		const response = await this.send({ type: "reset" });
+	async newSession(parentSession?: string): Promise<{ cancelled: boolean }> {
+		const response = await this.send({ type: "new_session", parentSession });
 		return this.getData(response);
 	}