Capture CLI stderr diagnostics

Goal

Capture agent CLI error signals without confusing them with model output.

Steps

1. Define a synchronous stderr_callback(message: str) and store or filter only necessary lines. [stderr]
2. Pass it with ClaudeAgentOptions(stderr=stderr_callback) to query(..., options=...). [stderr]
3. Temporarily record error class and time; for verbose logs, use a controlled debug-file as shown and define retention. [stderr]

How to verify

The stderr callback is invoked and its logs remain separate from AssistantMessage content.

Troubleshooting

If no lines appear, check SDK/CLI versions and whether the failure writes to stderr; no stderr does not imply success.