[kbn-evals] Evaluator discovery and preflight validation

### Summary

A typo in an evaluator name (`KibanaEvaluatorConfig(name="corectness")`) only surfaces at scoring time as `label="error"`, after the task has already run. Two methods for discovery and validation already exist in the API client but are wired to nothing.

### Problem

- `api/evaluators_client.py:48`: `list_evaluators()` exists but is not called in production code
- `api/evaluators_client.py:62`: `validate()` exists but is not called in production code
- `KibanaEvaluatorConfig.name` is a plain `str` with no validation; even an empty string is accepted
- No CLI command exists to list Kibana evaluators (a command does exist to list suites)

### Fix

- Validate evaluator names before the run starts using `list_evaluators()` or `validate()`
- Add an `elastic-evals list-evaluators` CLI command
- Wire this into the named classmethod approach (see #36)

### Done when

- [ ] A misspelt evaluator name raises a clear error before any task runs
- [ ] `elastic-evals list-evaluators` prints the evaluators available in Kibana