feat: add browser scenario test support

[Copilot]

Scenarios can now include an optional scenario.browser.test.ts alongside the required scenario.test.ts, running through Vitest’s Playwright browser mode.

• Discovery

  • Resolve and serialize the optional browser test path.
  • Keep grading tests excluded from the agent workspace.

• Execution

  • Validate Playwright dependencies and install Chromium before agent execution.
  • Run browser tests headlessly after node tests.

• Results

  • Combine node and browser outcomes into one score and artifact.
  • Preserve per-file test descriptions.

• Authoring

  • Document required scenario dependencies:

npm install --save-dev vitest playwright @vitest/browser-playwright

[joshblack]

@copilot don't make consumers install these deps. Just mirror what we already do for vitest and install them automatically