feat: Prepare agent-eval for npm CLI usage

[Copilot]

Prepares @primer/agent-eval for npm publishing as a runnable CLI, with support for running experiments from a caller-provided local folder while retaining typed experiment authoring.

• CLI package metadata

  • Adds agent-eval as the package binary.
  • Publishes built CLI output, config types, and Node engine metadata.
  • Bundles internal workspace code needed by the CLI and declares external runtime dependencies.

• Local experiment loading

  • Adds --experiments to load experiment files from a local directory.
  • Supports experiment named exports and default exports.
  • Falls back to packaged experiment lookup when no local directory is provided.

• Typed config authoring

  • Adds defineExperimentConfig from @primer/agent-eval/config.

import {defineExperimentConfig} from '@primer/agent-eval/config'

export const experiment = defineExperimentConfig({
  name: 'Example experiment',
  description: 'Compare treatment behavior',
  models: ['gpt-5.5'],
  evals: ['001-agent-uses-button-from-primer'],
  treatments: [],
})

COPILOT_GITHUB_TOKEN=... agent-eval --experiments ./experiments --experiment example