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/main.ts

@@ -449,17 +449,12 @@ export async function main(args: string[]) {
 		}
 	}
 
-	// Subscribe to agent events to save messages and log events
+	// Subscribe to agent events to save messages
 	agent.subscribe((event) => {
 		// Save messages on completion
 		if (event.type === "message_end") {
 			sessionManager.saveMessage(event.message);
 		}
-
-		// Log all events except message_update (too verbose)
-		if (event.type !== "message_update") {
-			sessionManager.saveEvent(event);
-		}
 	});
 
 	// Route to appropriate mode