Add optional OpenTelemetry span export

For teams that already use Jaeger/Datadog/Honeycomb, it would be useful to export trace steps as OpenTelemetry spans.

Should be opt-in (zero-dep by default):
```bash
pip install ai-decision-tracer[otel]
```

```python
tracer = Tracer("agent", exporters=[OTelExporter(endpoint="...")])
```

Local JSONL + Markdown output remains the default.