feat(coding): add core host api composition

electron/coding-runtime/pi/worker-pool.ts

@@ -1,5 +1,8 @@
-import type { PrepareConversationInput } from '../contracts';
-import type { ConversationModelState } from '../contracts';
+import type {
+  CodingRuntimeDiagnostics,
+  ConversationModelState,
+  PrepareConversationInput,
+} from '../contracts';
 import type { PiProcessError, PiProcessErrorCode } from './process-errors';
 import type {
   PiRpcCommand,
@@ -347,6 +350,27 @@ export class PiWorkerPool {
     return record ? this.publicState(record) : null;
   }
 
+  getDiagnostics(): CodingRuntimeDiagnostics {
+    const stage = (
+      state: PiWorkerPoolState['state'],
+    ): CodingRuntimeDiagnostics['workers'][number]['stage'] => {
+      if (state === 'spawning') return 'starting';
+      if (state === 'queued') return 'queued';
+      if (state === 'running') return 'running';
+      if (state === 'crashed') return 'failed';
+      return 'idle';
+    };
+    return {
+      revision: this.revisions.current,
+      workers: [...this.workers.values()].map((record) => ({
+        conversationId: record.conversation.conversationId,
+        generation: record.generation,
+        state: record.state,
+        stage: stage(record.state),
+      })),
+    };
+  }
+
   async reclaimIdleWorker(signal?: AbortSignal): Promise<boolean> {
     while (true) {
       if (signal?.aborted) throw new Error('Pi idle worker reclaim cancelled');