chore: add temporary P2 child debug channel for acceptance diagnosis

backend/src/workspaces/preprocessing-service.ts

@@ -427,7 +427,13 @@ export class WorkspacePreprocessingService {
 
   private async runJsonStage(runtime: ActiveRuntime, argv: string[]): Promise<Record<string, unknown>> {
     const result = await this.deps.runChild({ argv, configPath: runtime.configLease.path });
-    if (result.exitCode !== 0) throw new Error("workspace child failed");
+    if (result.exitCode !== 0) {
+      if (process.env.P2_CHILD_DEBUG === "1") {
+        const detail = `${result.stderr ?? ""}${result.stdout ?? ""}`.replace(/[\r\n]+/g, " ").slice(0, 2048);
+        throw new Error(`workspace child failed: ${detail}`);
+      }
+      throw new Error("workspace child failed");
+    }
     return JSON.parse(result.stdout) as Record<string, unknown>;
   }