[Code Quality] Add Antigravity engine to engines reference documentation

[github-actions]

Description

The antigravity engine is a fully supported runtime engine (pkg/workflow/antigravity_engine.go:24) used in production workflows (.github/workflows/smoke-antigravity.md:37), but it is missing from docs/src/content/docs/reference/engines.md. Users consulting the docs will not know this engine exists.

Suggested Changes

1. Add Antigravity to the available-engine table in docs/src/content/docs/reference/engines.md
2. Update schema-generated descriptions/examples in pkg/parser/schemas/main_workflow_schema.json so built-in engine coverage includes antigravity alongside pi and others
3. Add engine.extensions subsection to docs/src/content/docs/reference/frontmatter.md near engine.driver (currently documented in engines reference but missing from main frontmatter reference)

Files Affected

• docs/src/content/docs/reference/engines.md
• docs/src/content/docs/reference/frontmatter.md
• pkg/parser/schemas/main_workflow_schema.json (description/examples update)

Success Criteria

• Antigravity engine appears in the engines reference with description and example
• engine.extensions field is documented in frontmatter reference
• Schema examples/descriptions reflect the complete built-in engine catalog
• All existing doc build checks pass

Priority

Medium — valid production config is entirely undocumented for users

Source

Extracted from Schema Consistency Check #47499

🔍 Task mining by Discussion Task Miner - Code Quality Improvement Agent · sonnet46 · 42.2 AIC · ⌖ 5.29 AIC · ⊞ 7.1K · ◷

• expires on Jul 24, 2026, 11:12 AM UTC-08:00

[github-actions]

This issue was automatically closed because it expired on 2026-07-24T19:12:37.907Z.

Closed by Workflow