debug: add stderr callback to SDK engine for error visibility

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/services/agent-service/src/infrastructure/engines/claude-agent-sdk/claude-agent-sdk-engine.ts

@@ -109,6 +109,9 @@ export class ClaudeAgentSdkEngine implements AgentEnginePort {
           abortController,
           allowDangerouslySkipPermissions: true,
           permissionMode: 'bypassPermissions',
+          stderr: (data: string) => {
+            this.logger.debug(`SDK stderr (${params.sessionId}): ${data.trim()}`);
+          },
           canUseTool: async (toolName, toolInput, { signal }) => {
             const riskLevel = this.classifyToolRisk(toolName, toolInput);