[kbn-evals] Tracing not initialised: all trace IDs null

### Summary

`init_tracing()` is never called by the SDK. Without it, every ingested score has `trace_id=null`, and every Kibana evaluator short-circuits to `label="unavailable"`. Running an experiment with default settings produces no usable evaluation results.

### Problem

- `init_tracing()` is only called in example scripts, never from `run_experiment()`
- `evaluators/kibana.py:226-231` returns `"unavailable"` when `trace_id` is None
- Nothing in the docs warns users about this

### Fix

Call `init_tracing(config.tracing)` automatically in `ElasticEvalsClient.__init__`. `TracingConfig.enabled` already exists as the opt-out: setting `ELASTIC_EVALS_TRACING_ENABLED=false` in the environment disables tracing before the client is constructed. No new API parameter is needed.

### Done when

- [ ] Running `run_experiment()` without calling `init_tracing()` first does not produce `trace_id=null` scores
- [ ] The opt-out (`ELASTIC_EVALS_TRACING_ENABLED=false`) is documented