perf(plugins): keep turn-ended payloads independent of history (#7477)

Stop materializing and copying the entire conversation through lifecycle hooks. Keep the deprecated timeline field empty and direct plugins to bounded timeline reads.

packages/plugin/src/server/lifecycle.ts

@@ -50,6 +50,10 @@ export interface PluginLifecycleEvents {
     agent: PluginHookAgent;
     turnId: string | null;
     outcome: PluginTurnOutcome;
+    /**
+     * @deprecated Always empty. Fetch history on demand with
+     * context.paseo.agents.ref(agent.id).timeline.refetch({ direction: "tail", limit: 20 }).
+     */
     timeline: readonly AgentTimelineItem[];
   };
   "agent.permission_requested": { agent: PluginHookAgent; request: AgentPermissionRequest };