[kbn-evals] Replace evaluator factories with named classmethods

[milistu]

Summary

The factory functions have two problems. First, combining two factories of the same family (e.g. analysis + quantitative) silently doubles the Kibana judge API calls per trace because each factory creates its own _EvaluationBatch. Second, the four CODE evaluator factories are byte-for-byte identical except for three strings (docstring, function name, and the name= argument), split across eight files.

Proposed API

kibana_evaluators([
    KibanaEvaluatorConfig.correctness(connector_id="..."),
    KibanaEvaluatorConfig.groundedness(connector_id="..."),
    KibanaEvaluatorConfig.latency(),
    KibanaEvaluatorConfig.input_tokens(),
], client=client)

One kibana_evaluators() call = one HTTP request per trace, regardless of how many evaluators are in the list. IDE autocomplete on KibanaEvaluatorConfig. shows everything available. The double-cost problem is impossible by design.

Done when

• KibanaEvaluatorConfig has named classmethods for all built-in evaluator types
• The standalone create_* factory functions are removed
• README and evaluator reference show the composable API as the primary approach
• One kibana_evaluators() call with multiple configs produces one HTTP request per trace