Surface failed subagent tool calls with a resumable task_id

Fixes

• CoreRetry provider responses that end with finish_reason: network_error.
• CoreRetry more network error variants, including network-error and network_error.
• CoreSurface resumable subagent failures instead of returning an empty result.
• CorePreserve Cerebras max_completion_tokens without applying an extra output cap.
• CoreAnswer permission requests triggered by subagents during opencode run.
• CoreRetry xAI capacity and temporary unavailability stream errors.