feat(pi): add read-only subagent attach view

modules/agents/pi/extensions/subagents/supervisor.ts

@@ -2,6 +2,7 @@ import type {
   ChildHandle,
   ChildRecord,
   ChildRunner,
+  SubagentActivityEvent,
   ContextMode,
   RunnerActivity,
   RunnerEvents,
@@ -323,7 +324,7 @@ export class Supervisor {
     return [...this.children.values()].find((child) => child.record === record);
   }
 
-  activity(id: string) {
+  activity(id: string): SubagentActivityEvent[] {
     return this.require(id).record.activityEvents.map((event) => ({ ...event }));
   }