Validate one or more scenario files for schema correctness.

Checks YAML syntax, required fields, gate configuration, regex compilation, and judge setup. No fixture setup, no LLM spend, no agent execution.

With --scenario, the given file is always validated regardless of whether it looks like a scenario.
With --all, YAML files are scanned recursively and a lightweight heuristic (at least two distinctive scenario keys: name, target, task, evaluation, or template_folder) is used to skip non-scenario files such as rubrics. This matches the discovery logic used by `run --all`.

Usage: ax-eval validate [OPTIONS]

Options:
  -s, --scenario <SCENARIO>
          Path to scenario file or name (always validated, even if not scenario-like)

      --all
          Validate all scenario-like YAML files in fixtures directory

  -v, --verbose
          Enable verbose output (or set RUST_LOG for fine-grained control)

  -h, --help
          Print help (see a summary with '-h')

Examples:
  ax-eval validate --scenario fixtures/my_scenario.yaml
  ax-eval validate --all
  ax-eval validate --scenario fixtures/my_scenario.yaml --verbose
