feat: add Pi subagent scheduler and child runtime

electron/coding-runtime/pi/runtime.ts

@@ -75,6 +75,7 @@ import {
   projectPiSessionSnapshot,
 } from './session-projector';
 import { PiManagedExtensionHost } from './extension-host';
+import type { PiSubagentScheduler } from './subagent';
 import {
   PiInteractionStore,
   type PiInteractionResponse,
@@ -98,6 +99,7 @@ export interface PiConversationRuntimeOptions {
   isAuthenticationError?(error: unknown): boolean;
   refreshCredential?(accountId: string): Promise<void>;
   extensionHost?: PiManagedExtensionHost;
+  subagentScheduler?: PiSubagentScheduler;
   getDraftRevision?(conversationId: string): number;
   knownExtensionWidgetKeys?: readonly string[];
   onExtensionUiProjection?(projection: PiExtensionUiProjection): void;
@@ -480,6 +482,15 @@ export class PiConversationRuntime implements CodingConversationRuntime {
     this.isAuthenticationError = options.isAuthenticationError;
     this.refreshCredential = options.refreshCredential;
     this.extensionHost = options.extensionHost;
+    if (options.subagentScheduler && !this.extensionHost) {
+      throw new Error('Subagent scheduler requires the managed extension host');
+    }
+    if (options.subagentScheduler) {
+      this.extensionHost?.configureSubagents({
+        scheduler: options.subagentScheduler,
+        trackGenerationResource: (input) => this.pool.trackGenerationResource(input),
+      });
+    }
     this.interactions = new PiInteractionStore(this.pool, (interaction) => {
       this.emit(interaction.conversationId, { op: 'interaction.upsert', interaction }, interaction.runId);
     });