Document Antigravity engine support in reference docs and schema guidance

[Copilot]

The reference docs and schema guidance listed a narrower engine set than the implementation actually supports: antigravity was live in code and workflows, but absent from engines.md and $defs.engine_config examples/descriptions. This updates the documentation surfaces that users and tooling rely on so Antigravity appears as a supported engine.

• Engine reference

  • Added Antigravity to the supported engine table in reference/engines.md
  • Added a focused Antigravity section with secret/config guidance
  • Included Antigravity in the version matrix and feature comparison table

• Schema guidance

  • Added antigravity to pkg/parser/schemas/main_workflow_schema.json $defs.engine_config
  • Updated both the string-form engine description and id description to include antigravity
  • Added an antigravity object example alongside the existing engine examples

• Auth reference

  • Added ANTIGRAVITY_API_KEY to reference/auth.mdx so the new engine docs link to a concrete secret reference instead of a missing anchor

• Reference consistency

  • Corrected the bare-mode comparison row in engines.md to match current engine capabilities while touching the engine matrix

engine:
  id: antigravity
  version: "1.0.2-6113393518706688"