feat: detect scenario test dependencies

[Copilot]

Scenario packages now declare their test runner dependency. Agent eval detects Vitest while keeping grading files hidden from the agent workspace.

• Runner detection

  • Detect Vitest in dependencies or devDependencies.
  • Use the detected runner for grading.

• Test isolation

  • Exclude scenario.test.ts and scenario.browser.test.ts from the initial workspace copy.

• Authoring

  • Document the Vitest dependency requirement.
  • Add a patch changeset.