v0.5.0: per-upstream retries, OpenAI client wrapper Major changes: - Per-upstream maxRetries and retryDelay with configurable retry delay function - taskEnvelope payload format wrapping RequestPayload + RetryDelay - X-Proxq-Source header on all proxq-generated responses - pkg/clients/openai: drop-in OpenAI SDK client routing through proxq async queue - pkg/types: shared public constants for header names