Add OTEL exporter example (Confident/Langfuse)

[christso]

Adds a standalone example that exports AgentV JSONL results to OTLP/HTTP traces for observability backends.

What's included:

• New example: examples/features/otel-export
• Export script: bun run export --in <results.jsonl> [--backend confident|langfuse]
• Backend support:
  • Confident AI via x-confident-api-key
  • Langfuse via OTLP /api/public/otel + Basic Auth
• Safe-by-default payload: only minimal metadata (eval_id/dataset/target/score/trace_summary), no prompts or tool I/O

Why:

• Keeps telemetry export out of AgentV core (consumer-side), while still enabling Confident/Langfuse ingestion using standard OpenTelemetry exporters.