Remove event logging from session files - only save messages and state changes

Session reconstruction only needs:
- type: 'session' - session metadata
- type: 'message' - conversation history
- type: 'thinking_level_change' - thinking level changes
- type: 'model_change' - model changes

Events like agent_start/end, tool_execution_start/end are not needed for
session reconstruction and only added bloat. This reduces session file size
significantly and speeds up writes.

packages/coding-agent/src/session-manager.ts

@@ -1,4 +1,4 @@
-import type { AgentEvent, AgentState } from "@mariozechner/pi-agent";
+import type { AgentState } from "@mariozechner/pi-agent";
 import { randomBytes } from "crypto";
 import { appendFileSync, existsSync, mkdirSync, readdirSync, readFileSync, statSync } from "fs";
 import { homedir } from "os";
@@ -27,12 +27,6 @@ export interface SessionMessageEntry {
 	message: any; // AppMessage from agent state
 }
 
-export interface SessionEventEntry {
-	type: "event";
-	timestamp: string;
-	event: AgentEvent;
-}
-
 export interface ThinkingLevelChangeEntry {
 	type: "thinking_level_change";
 	timestamp: string;
@@ -152,16 +146,6 @@ export class SessionManager {
 		appendFileSync(this.sessionFile, JSON.stringify(entry) + "\n");
 	}
 
-	saveEvent(event: AgentEvent): void {
-		if (!this.enabled) return;
-		const entry: SessionEventEntry = {
-			type: "event",
-			timestamp: new Date().toISOString(),
-			event,
-		};
-		appendFileSync(this.sessionFile, JSON.stringify(entry) + "\n");
-	}
-
 	saveThinkingLevelChange(thinkingLevel: string): void {
 		if (!this.enabled) return;
 		const entry: ThinkingLevelChangeEntry = {