chore: add temporary P2 child debug channel for acceptance diagnosis

backend/src/workspace-maintenance.ts

@@ -249,8 +249,15 @@ function createProductionService(): WorkspacePreprocessingService {
           let stderr = "";
           child.stdout?.on("data", (chunk: Buffer) => { stdout += chunk.toString("utf8"); });
           child.stderr?.on("data", (chunk: Buffer) => { stderr += chunk.toString("utf8"); });
-          child.on("close", (code) => resolve({ exitCode: code ?? 0, stdout, stderr }));
-          child.on("error", (error) => resolve({ exitCode: 1, stdout, stderr: String(error.message) }));
+          child.on("close", (code) => {
+            const capped = { exitCode: code ?? 0, stdout, stderr: stderr.slice(0, 64 * 1024) };
+            if (process.env.P2_CHILD_DEBUG === "1" && (code ?? 0) !== 0) {
+              capped.stdout = stdout.slice(0, 4096);
+              capped.stderr = stderr.slice(0, 4096);
+            }
+            resolve(capped);
+          });
+          child.on("error", (error) => resolve({ exitCode: 1, stdout, stderr: String(error.message).slice(0, 4096) }));
         });
       } finally {
         closeSync(configFd);