Document engine.token-weights and engine.extensions in the concise frontmatter reference

[Copilot]

The concise frontmatter reference documented engine: only as a simple selector, while runtime and the full reference also support engine.token-weights and engine.extensions. This updates the summary page so the short and full references describe the same supported engine capabilities.

• Reference alignment

  • Extend the engine: section in docs/src/content/docs/reference/frontmatter.md to call out object-form engine configuration, not just string form.
  • Explicitly mention:
    • token-weights for custom AI Credits cost ratios
    • extensions for engine-specific plugin installation

• Example update

  • Add a compact object-form engine: example that shows both fields in context.

engine:
  id: pi
  token-weights:
    multipliers:
      pi-medium: 0.5
  extensions:
    - acme/pi-search