Add OpenTelemetry observability for retrieval calls

[varunreddy]

Summary

Instrument the retrieval and emission paths with OpenTelemetry spans so teams can trace routing decisions in their observability stack.

Acceptance Criteria

• Add optional opentelemetry-api dependency (in an [otel] extra)
• Instrument retriever.py with spans: query received, BM25 score, dense score, RRF merge, top-K selection
• Instrument adapters/ with spans: context formatting, token count estimation
• Add span attributes: query text, registry name, top-K IDs, total cards searched
• No-op gracefully when OpenTelemetry is not installed
• Add a test verifying spans are emitted when the SDK is present
• Document setup in README or a dedicated docs/observability.md