fix: gate workspace diagnostic migration

backend/src/workspaces/diagnostics.ts

@@ -5,7 +5,7 @@ import { once } from "node:events";
 import { MAX_WORKSPACE_DIAGNOSTIC_TIMEOUT_MS } from "../config.js";
 import { buildInstallationContract } from "./contracts.js";
 import type { RuntimeBindings } from "./runtime-renderer.js";
-import { validateCanonicalWorkspace, type CanonicalWorkspace } from "./schema.js";
+import { validateCanonicalWorkspace, type CanonicalWorkspace, type WorkspaceDescriptor } from "./schema.js";
 import type { WorkspaceErrorCode } from "./types.js";
 
 export interface Diagnostic {
@@ -388,7 +388,7 @@ export function createWorkspaceDiagnoser(
   const fallbackTimeout = boundedTimeout(options.timeoutMs, DEFAULT_WORKSPACE_DIAGNOSTIC_TIMEOUT_MS);
 
   return async function diagnoseWorkspace(
-    workspace: CanonicalWorkspace,
+    workspace: WorkspaceDescriptor,
     bindings: RuntimeBindings,
     options: { writeProbe: boolean },
   ): Promise<WorkspaceDiagnostics> {